diff --git a/mail_intake/__init__.py b/mail_intake/__init__.py
index e69de29..0db787b 100644
--- a/mail_intake/__init__.py
+++ b/mail_intake/__init__.py
@@ -0,0 +1,126 @@
+"""
+Mail Intake — provider-agnostic, read-only email ingestion framework.
+
+Mail Intake is a **contract-first library** designed to ingest, parse, and
+normalize email data from external providers (such as Gmail) into clean,
+provider-agnostic domain models.
+
+The library is intentionally structured around clear layers, each exposed
+as a first-class module at the package root:
+
+- adapters: provider-specific access (e.g. Gmail)
+- auth: authentication providers and credential management
+- parsers: extraction and normalization of message content
+- ingestion: orchestration and high-level ingestion workflows
+- models: canonical, provider-agnostic data representations
+- config: explicit global configuration
+- exceptions: library-defined error hierarchy
+
+The package root acts as a **namespace**, not a facade. Consumers are
+expected to import functionality explicitly from the appropriate module.
+
+----------------------------------------------------------------------
+Installation
+----------------------------------------------------------------------
+
+Install using pip:
+
+    pip install mail-intake
+
+Or with Poetry:
+
+    poetry add mail-intake
+
+Mail Intake is pure Python and has no runtime dependencies beyond those
+required by the selected provider (for example, Google APIs for Gmail).
+
+----------------------------------------------------------------------
+Basic Usage
+----------------------------------------------------------------------
+
+Minimal Gmail ingestion example:
+
+    from mail_intake.ingestion import MailIntakeReader
+    from mail_intake.adapters import MailIntakeGmailAdapter
+    from mail_intake.auth import MailIntakeGoogleAuth
+
+    auth = MailIntakeGoogleAuth(
+        credentials_path="credentials.json",
+        token_path="token.pickle",
+        scopes=["https://www.googleapis.com/auth/gmail.readonly"],
+    )
+
+    adapter = MailIntakeGmailAdapter(auth_provider=auth)
+    reader = MailIntakeReader(adapter)
+
+    for message in reader.iter_messages("from:recruiter@example.com"):
+        print(message.subject, message.from_email)
+
+Iterating over threads:
+
+    for thread in reader.iter_threads("subject:Interview"):
+        print(thread.normalized_subject, len(thread.messages))
+
+----------------------------------------------------------------------
+Extensibility Model
+----------------------------------------------------------------------
+
+Mail Intake is designed to be extensible via **public contracts** exposed
+through its modules:
+
+- Users MAY implement their own mail adapters by subclassing
+  `adapters.MailIntakeAdapter`
+- Users MAY implement their own authentication providers by subclassing
+  `auth.MailIntakeAuthProvider`
+
+Users SHOULD NOT subclass built-in adapter implementations. Built-in
+adapters (such as Gmail) are reference implementations and may change
+internally without notice.
+
+----------------------------------------------------------------------
+Public API Surface
+----------------------------------------------------------------------
+
+The supported public API consists of the following top-level modules:
+
+- mail_intake.ingestion
+- mail_intake.adapters
+- mail_intake.auth
+- mail_intake.parsers
+- mail_intake.models
+- mail_intake.config
+- mail_intake.exceptions
+
+Classes and functions should be imported explicitly from these modules.
+No individual symbols are re-exported at the package root.
+
+----------------------------------------------------------------------
+Design Guarantees
+----------------------------------------------------------------------
+
+- Read-only access: no mutation of provider state
+- Provider-agnostic domain models
+- Explicit configuration and dependency injection
+- No implicit global state or environment reads
+- Deterministic, testable behavior
+
+Mail Intake favors correctness, clarity, and explicitness over convenience
+shortcuts.
+"""
+
+
+import ingestion
+import adapters
+import auth
+import models
+import config
+import exceptions
+
+__all__ = [
+    "ingestion",
+    "adapters",
+    "auth",
+    "models",
+    "config",
+    "exceptions",
+]
diff --git a/mail_intake/adapters/__init__.py b/mail_intake/adapters/__init__.py
index e69de29..908d78c 100644
--- a/mail_intake/adapters/__init__.py
+++ b/mail_intake/adapters/__init__.py
@@ -0,0 +1,28 @@
+"""
+Mail provider adapter implementations for Mail Intake.
+
+This package contains **adapter-layer implementations** responsible for
+interfacing with external mail providers and exposing a normalized,
+provider-agnostic contract to the rest of the system.
+
+Adapters in this package:
+- Implement the `MailIntakeAdapter` interface
+- Encapsulate all provider-specific APIs and semantics
+- Perform read-only access to mail data
+- Return provider-native payloads without interpretation
+
+Provider-specific logic **must not leak** outside of adapter implementations.
+All parsings, normalizations, and transformations must be handled by downstream
+components.
+
+Public adapters exported from this package are considered the supported
+integration surface for mail providers.
+"""
+
+from .base import MailIntakeAdapter
+from .gmail import MailIntakeGmailAdapter
+
+__all__ = [
+    "MailIntakeAdapter",
+    "MailIntakeGmailAdapter",
+]
diff --git a/mail_intake/adapters/base.py b/mail_intake/adapters/base.py
index e26e17d..f956e75 100644
--- a/mail_intake/adapters/base.py
+++ b/mail_intake/adapters/base.py
@@ -1,3 +1,14 @@
+"""
+Mail provider adapter contracts for Mail Intake.
+
+This module defines the **provider-agnostic adapter interface** used for
+read-only mail ingestion.
+
+Adapters encapsulate all provider-specific access logic and expose a
+minimal, normalized contract to the rest of the system. No provider-specific
+types or semantics should leak beyond implementations of this interface.
+"""
+
 from abc import ABC, abstractmethod
 from typing import Iterator, Dict, Any
 
@@ -6,43 +17,60 @@ class MailIntakeAdapter(ABC):
     """
     Base adapter interface for mail providers.
 
-    This interface defines the minimal contract required for
-    read-only mail ingestion. No provider-specific concepts
-    should leak beyond implementations of this class.
+    This interface defines the minimal contract required to:
+    - Discover messages matching a query
+    - Retrieve full message payloads
+    - Retrieve full thread payloads
+
+    Adapters are intentionally read-only and must not mutate provider state.
     """
 
     @abstractmethod
     def iter_message_refs(self, query: str) -> Iterator[Dict[str, str]]:
         """
-        Iterate over lightweight message references.
+        Iterate over lightweight message references matching a query.
 
-        Must yield dictionaries containing at least:
-        - message_id
-        - thread_id
+        Implementations must yield dictionaries containing at least:
+        - ``message_id``: Provider-specific message identifier
+        - ``thread_id``: Provider-specific thread identifier
+
+        Args:
+            query: Provider-specific query string used to filter messages.
+
+        Yields:
+            Dictionaries containing message and thread identifiers.
 
         Example yield:
-        {
-            "message_id": "...",
-            "thread_id": "..."
-        }
+            {
+                "message_id": "...",
+                "thread_id": "..."
+            }
         """
         raise NotImplementedError
 
     @abstractmethod
     def fetch_message(self, message_id: str) -> Dict[str, Any]:
         """
-        Fetch a full raw message by message_id.
+        Fetch a full raw message by message identifier.
 
-        Returns the provider-native message payload
-        (e.g., Gmail message JSON).
+        Args:
+            message_id: Provider-specific message identifier.
+
+        Returns:
+            Provider-native message payload
+            (e.g., Gmail message JSON structure).
         """
         raise NotImplementedError
 
     @abstractmethod
     def fetch_thread(self, thread_id: str) -> Dict[str, Any]:
         """
-        Fetch a full raw thread by thread_id.
+        Fetch a full raw thread by thread identifier.
 
-        Returns the provider-native thread payload.
+        Args:
+            thread_id: Provider-specific thread identifier.
+
+        Returns:
+            Provider-native thread payload.
         """
         raise NotImplementedError
diff --git a/mail_intake/adapters/gmail.py b/mail_intake/adapters/gmail.py
index d4c52dc..969dc86 100644
--- a/mail_intake/adapters/gmail.py
+++ b/mail_intake/adapters/gmail.py
@@ -1,3 +1,17 @@
+"""
+Gmail adapter implementation for Mail Intake.
+
+This module provides a **Gmail-specific implementation** of the
+`MailIntakeAdapter` contract.
+
+It is the only place in the codebase where:
+- `googleapiclient` is imported
+- Gmail REST API semantics are known
+- Low-level `.execute()` calls are made
+
+All Gmail-specific behavior must be strictly contained within this module.
+"""
+
 from typing import Iterator, Dict, Any
 
 from googleapiclient.discovery import build
@@ -12,12 +26,19 @@ class MailIntakeGmailAdapter(MailIntakeAdapter):
     """
     Gmail read-only adapter.
 
+    This adapter implements the `MailIntakeAdapter` interface using the
+    Gmail REST API. It translates the generic mail intake contract into
+    Gmail-specific API calls.
+
     This class is the ONLY place where:
     - googleapiclient is imported
     - Gmail REST semantics are known
     - .execute() is called
 
-    It must remain thin and dumb by design.
+    Design constraints:
+    - Must remain thin and imperative
+    - Must not perform parsing or interpretation
+    - Must not expose Gmail-specific types beyond this class
     """
 
     def __init__(
@@ -25,12 +46,29 @@ class MailIntakeGmailAdapter(MailIntakeAdapter):
         auth_provider: MailIntakeAuthProvider,
         user_id: str = "me",
     ):
+        """
+        Initialize the Gmail adapter.
+
+        Args:
+            auth_provider: Authentication provider capable of supplying
+                valid Gmail API credentials.
+            user_id: Gmail user identifier. Defaults to `"me"`.
+        """
         self._auth_provider = auth_provider
         self._user_id = user_id
         self._service = None
 
     @property
     def service(self):
+        """
+        Lazily initialize and return the Gmail API service client.
+
+        Returns:
+            Initialized Gmail API service instance.
+
+        Raises:
+            MailIntakeAdapterError: If the Gmail service cannot be initialized.
+        """
         if self._service is None:
             try:
                 creds = self._auth_provider.get_credentials()
@@ -45,11 +83,16 @@ class MailIntakeGmailAdapter(MailIntakeAdapter):
         """
         Iterate over message references matching the query.
 
+        Args:
+            query: Gmail search query string.
+
         Yields:
-        {
-            "message_id": "...",
-            "thread_id": "..."
-        }
+            Dictionaries containing:
+            - ``message_id``: Gmail message ID
+            - ``thread_id``: Gmail thread ID
+
+        Raises:
+            MailIntakeAdapterError: If the Gmail API returns an error.
         """
         try:
             request = (
@@ -79,6 +122,18 @@ class MailIntakeGmailAdapter(MailIntakeAdapter):
             ) from exc
 
     def fetch_message(self, message_id: str) -> Dict[str, Any]:
+        """
+        Fetch a full Gmail message by message ID.
+
+        Args:
+            message_id: Gmail message identifier.
+
+        Returns:
+            Provider-native Gmail message payload.
+
+        Raises:
+            MailIntakeAdapterError: If the Gmail API returns an error.
+        """
         try:
             return (
                 self.service.users()
@@ -92,6 +147,18 @@ class MailIntakeGmailAdapter(MailIntakeAdapter):
             ) from exc
 
     def fetch_thread(self, thread_id: str) -> Dict[str, Any]:
+        """
+        Fetch a full Gmail thread by thread ID.
+
+        Args:
+            thread_id: Gmail thread identifier.
+
+        Returns:
+            Provider-native Gmail thread payload.
+
+        Raises:
+            MailIntakeAdapterError: If the Gmail API returns an error.
+        """
         try:
             return (
                 self.service.users()
diff --git a/mail_intake/auth/__init__.py b/mail_intake/auth/__init__.py
index e69de29..342f482 100644
--- a/mail_intake/auth/__init__.py
+++ b/mail_intake/auth/__init__.py
@@ -0,0 +1,26 @@
+"""
+Authentication provider implementations for Mail Intake.
+
+This package defines the **authentication layer** used by mail adapters
+to obtain provider-specific credentials.
+
+It exposes:
+- A stable, provider-agnostic authentication contract
+- Concrete authentication providers for supported platforms
+
+Authentication providers:
+- Are responsible for credential acquisition and lifecycle management
+- Are intentionally decoupled from adapter logic
+- May be extended by users to support additional providers
+
+Consumers should depend on the abstract interface and use concrete
+implementations only where explicitly required.
+"""
+
+from .base import MailIntakeAuthProvider
+from .google import MailIntakeGoogleAuth
+
+__all__ = [
+    "MailIntakeAuthProvider",
+    "MailIntakeGoogleAuth",
+]
diff --git a/mail_intake/auth/base.py b/mail_intake/auth/base.py
index 526ff36..24cfcdb 100644
--- a/mail_intake/auth/base.py
+++ b/mail_intake/auth/base.py
@@ -1,3 +1,14 @@
+"""
+Authentication provider contracts for Mail Intake.
+
+This module defines the **authentication abstraction layer** used by mail
+adapters to obtain provider-specific credentials.
+
+Authentication concerns are intentionally decoupled from adapter logic.
+Adapters depend only on this interface and must not be aware of how
+credentials are acquired, refreshed, or stored.
+"""
+
 from abc import ABC, abstractmethod
 
 
@@ -7,6 +18,17 @@ class MailIntakeAuthProvider(ABC):
 
     Mail adapters depend on this interface, not on concrete
     OAuth or credential implementations.
+
+    Authentication providers encapsulate all logic required to acquire
+    valid credentials for a mail provider.
+
+    Implementations may involve:
+    - OAuth flows
+    - Service account credentials
+    - Token refresh logic
+    - Secure credential storage
+
+    Adapters must treat the returned credentials as opaque and provider-specific.
     """
 
     @abstractmethod
@@ -16,5 +38,13 @@ class MailIntakeAuthProvider(ABC):
 
         This method is synchronous by design and must either
         return valid credentials or raise MailIntakeAuthError.
+
+        Returns:
+            Provider-specific credentials object suitable for use by
+            the corresponding mail adapter.
+
+        Raises:
+            Exception: Authentication-specific errors defined by the
+                implementation.
         """
         raise NotImplementedError
diff --git a/mail_intake/auth/google.py b/mail_intake/auth/google.py
index ec9a4f1..a6dcc3e 100644
--- a/mail_intake/auth/google.py
+++ b/mail_intake/auth/google.py
@@ -1,3 +1,17 @@
+"""
+Google authentication provider implementation for Mail Intake.
+
+This module provides a **Google OAuth–based authentication provider**
+used primarily for Gmail access.
+
+It encapsulates all Google-specific authentication concerns, including:
+- Credential loading and persistence
+- Token refresh handling
+- Interactive OAuth flow initiation
+
+No Google authentication details should leak outside this module.
+"""
+
 import os
 import pickle
 from typing import Sequence
@@ -14,12 +28,16 @@ class MailIntakeGoogleAuth(MailIntakeAuthProvider):
     """
     Google OAuth provider for Gmail access.
 
-    Responsibilities:
-    - Load cached credentials from disk
-    - Refresh expired tokens when possible
-    - Trigger interactive login only when strictly required
+    This provider implements the `MailIntakeAuthProvider` interface using
+    Google's OAuth 2.0 flow and credential management libraries.
 
-    This class is synchronous and intentionally state-light.
+    Responsibilities:
+    - Load cached credentials from disk when available
+    - Refresh expired credentials when possible
+    - Initiate an interactive OAuth flow only when required
+    - Persist refreshed or newly obtained credentials
+
+    This class is synchronous by design and maintains a minimal internal state.
     """
 
     def __init__(
@@ -28,11 +46,36 @@ class MailIntakeGoogleAuth(MailIntakeAuthProvider):
         token_path: str,
         scopes: Sequence[str],
     ):
+        """
+        Initialize the Google authentication provider.
+
+        Args:
+            credentials_path: Path to the Google OAuth client secrets file.
+            token_path: Path where OAuth tokens will be cached.
+            scopes: OAuth scopes required for access.
+        """
         self.credentials_path = credentials_path
         self.token_path = token_path
         self.scopes = list(scopes)
 
     def get_credentials(self):
+        """
+        Retrieve valid Google OAuth credentials.
+
+        This method attempts to:
+        1. Load cached credentials from disk
+        2. Refresh expired credentials when possible
+        3. Perform an interactive OAuth login as a fallback
+        4. Persist valid credentials for future use
+
+        Returns:
+            Google OAuth credentials object suitable for use with
+            Google API clients.
+
+        Raises:
+            MailIntakeAuthError: If credentials cannot be loaded, refreshed,
+                or obtained via interactive authentication.
+        """
         creds = None
 
         # Attempt to load cached credentials
diff --git a/mail_intake/config.py b/mail_intake/config.py
index afd63c5..6898ceb 100644
--- a/mail_intake/config.py
+++ b/mail_intake/config.py
@@ -1,3 +1,14 @@
+"""
+Global configuration models for Mail Intake.
+
+This module defines the **top-level configuration object** used to control
+mail ingestion behavior across adapters, authentication providers, and
+ingestion workflows.
+
+Configuration is intentionally explicit, immutable, and free of implicit
+environment reads to ensure predictability and testability.
+"""
+
 from dataclasses import dataclass
 from typing import Optional
 
@@ -9,12 +20,26 @@ class MailIntakeConfig:
 
     This configuration is intentionally explicit and immutable.
     No implicit environment reads or global state.
+
+    Design principles:
+    - Immutable once constructed
+    - Explicit configuration over implicit defaults
+    - No direct environment or filesystem access
+
+    This model is safe to pass across layers and suitable for serialization.
     """
 
     provider: str = "gmail"
-    user_id: str = "me"
-    readonly: bool = True
+    """Identifier of the mail provider to use (e.g., ``"gmail"``)."""
+
+    user_id: str = "me"
+    """Provider-specific user identifier. Defaults to the authenticated user."""
+
+    readonly: bool = True
+    """Whether ingestion should operate in read-only mode."""
 
-    # Provider-specific paths (optional at this layer)
     credentials_path: Optional[str] = None
+    """Optional path to provider credentials configuration."""
+
     token_path: Optional[str] = None
+    """Optional path to persisted authentication tokens."""
diff --git a/mail_intake/exceptions.py b/mail_intake/exceptions.py
index 43043f4..5bba3fb 100644
--- a/mail_intake/exceptions.py
+++ b/mail_intake/exceptions.py
@@ -1,19 +1,49 @@
+"""
+Exception hierarchy for Mail Intake.
+
+This module defines the **canonical exception types** used throughout the
+Mail Intake library.
+
+All library-raised errors derive from `MailIntakeError`. Consumers are
+encouraged to catch this base type (or specific subclasses) rather than
+provider-specific or third-party exceptions.
+"""
+
+
 class MailIntakeError(Exception):
     """
-    Base exception for all mail-intake errors.
+    Base exception for all Mail Intake errors.
 
-    Users of the library should catch this type (or subclasses)
-    instead of provider-specific or third-party exceptions.
+    This is the root of the Mail Intake exception hierarchy.
+    All errors raised by the library must derive from this class.
+
+    Consumers should generally catch this type when handling
+    library-level failures.
     """
 
 
 class MailIntakeAuthError(MailIntakeError):
-    """Authentication and credential-related failures."""
+    """
+    Authentication and credential-related failures.
+
+    Raised when authentication providers are unable to acquire,
+    refresh, or persist valid credentials.
+    """
 
 
 class MailIntakeAdapterError(MailIntakeError):
-    """Errors raised by mail provider adapters."""
+    """
+    Errors raised by mail provider adapters.
+
+    Raised when a provider adapter encounters API errors,
+    transport failures, or invalid provider responses.
+    """
 
 
 class MailIntakeParsingError(MailIntakeError):
-    """Errors encountered while parsing message content."""
+    """
+    Errors encountered while parsing message content.
+
+    Raised when raw provider payloads cannot be interpreted
+    or normalized into internal domain models.
+    """
diff --git a/mail_intake/ingestion/__init__.py b/mail_intake/ingestion/__init__.py
index e69de29..54b10ac 100644
--- a/mail_intake/ingestion/__init__.py
+++ b/mail_intake/ingestion/__init__.py
@@ -0,0 +1,24 @@
+"""
+Mail ingestion orchestration for Mail Intake.
+
+This package contains **high-level ingestion components** responsible for
+coordinating mail retrieval, parsing, normalization, and model construction.
+
+It represents the **top of the ingestion pipeline** and is intended to be the
+primary interaction surface for library consumers.
+
+Components in this package:
+- Are provider-agnostic
+- Depend only on adapter and parser contracts
+- Contain no provider-specific API logic
+- Expose read-only ingestion workflows
+
+Consumers are expected to construct a mail adapter and pass it to the
+ingestion layer to begin processing messages and threads.
+"""
+
+from .reader import MailIntakeReader
+
+__all__ = [
+    "MailIntakeReader",
+]
diff --git a/mail_intake/ingestion/reader.py b/mail_intake/ingestion/reader.py
index 186d296..6bec5b8 100644
--- a/mail_intake/ingestion/reader.py
+++ b/mail_intake/ingestion/reader.py
@@ -1,3 +1,18 @@
+"""
+High-level mail ingestion orchestration for Mail Intake.
+
+This module provides the primary, provider-agnostic entry point for
+reading and processing mail data.
+
+It coordinates:
+- Mail adapter access
+- Message and thread iteration
+- Header and body parsing
+- Normalization and model construction
+
+No provider-specific logic or API semantics are permitted in this layer.
+"""
+
 from datetime import datetime
 from typing import Iterator, Dict, Any
 
@@ -14,22 +29,43 @@ class MailIntakeReader:
     """
     High-level read-only ingestion interface.
 
-    This is the primary entry point users should interact with.
-    It orchestrates:
-    - adapter calls
-    - parsing
-    - normalization
-    - model construction
+    This class is the **primary entry point** for consumers of the Mail
+    Intake library.
 
-    No provider-specific logic exists here.
+    It orchestrates the full ingestion pipeline:
+    - Querying the adapter for message references
+    - Fetching raw provider messages
+    - Parsing and normalizing message data
+    - Constructing domain models
+
+    This class is intentionally:
+    - Provider-agnostic
+    - Stateless beyond iteration scope
+    - Read-only
     """
 
     def __init__(self, adapter: MailIntakeAdapter):
+        """
+        Initialize the mail reader.
+
+        Args:
+            adapter: Mail adapter implementation used to retrieve raw
+                messages and threads from a mail provider.
+        """
         self._adapter = adapter
 
     def iter_messages(self, query: str) -> Iterator[MailIntakeMessage]:
         """
         Iterate over parsed messages matching a provider query.
+
+        Args:
+            query: Provider-specific query string used to filter messages.
+
+        Yields:
+            Fully parsed and normalized `MailIntakeMessage` instances.
+
+        Raises:
+            MailIntakeParsingError: If a message cannot be parsed.
         """
         for ref in self._adapter.iter_message_refs(query):
             raw = self._adapter.fetch_message(ref["message_id"])
@@ -39,7 +75,17 @@ class MailIntakeReader:
         """
         Iterate over threads constructed from messages matching a query.
 
-        Messages are grouped by thread_id and yielded as complete threads.
+        Messages are grouped by `thread_id` and yielded as complete thread
+        objects containing all associated messages.
+
+        Args:
+            query: Provider-specific query string used to filter messages.
+
+        Returns:
+            An iterator of `MailIntakeThread` instances.
+
+        Raises:
+            MailIntakeParsingError: If a message cannot be parsed.
         """
         threads: Dict[str, MailIntakeThread] = {}
 
@@ -61,7 +107,17 @@ class MailIntakeReader:
 
     def _parse_message(self, raw_message: Dict[str, Any]) -> MailIntakeMessage:
         """
-        Parse a raw provider message into a MailIntakeMessage.
+        Parse a raw provider message into a `MailIntakeMessage`.
+
+        Args:
+            raw_message: Provider-native message payload.
+
+        Returns:
+            A fully populated `MailIntakeMessage` instance.
+
+        Raises:
+            MailIntakeParsingError: If the message payload is missing required
+                fields or cannot be parsed.
         """
         try:
             message_id = raw_message["id"]
diff --git a/mail_intake/models/__init__.py b/mail_intake/models/__init__.py
index e69de29..47d17c3 100644
--- a/mail_intake/models/__init__.py
+++ b/mail_intake/models/__init__.py
@@ -0,0 +1,22 @@
+"""
+Domain models for Mail Intake.
+
+This package defines the **canonical, provider-agnostic data models**
+used throughout the Mail Intake ingestion pipeline.
+
+Models in this package:
+- Represent fully parsed and normalized mail data
+- Are safe to persist, serialize, and index
+- Contain no provider-specific payloads or API semantics
+- Serve as stable inputs for downstream processing and analysis
+
+These models form the core internal data contract of the library.
+"""
+
+from .message import MailIntakeMessage
+from .thread import MailIntakeThread
+
+__all__ = [
+    "MailIntakeMessage",
+    "MailIntakeThread",
+]
diff --git a/mail_intake/models/message.py b/mail_intake/models/message.py
index 2696d75..115b4eb 100644
--- a/mail_intake/models/message.py
+++ b/mail_intake/models/message.py
@@ -1,3 +1,14 @@
+"""
+Message domain models for Mail Intake.
+
+This module defines the **canonical, provider-agnostic representation**
+of an individual email message as used internally by the Mail Intake
+ingestion pipeline.
+
+Models in this module are safe to persist and must not contain any
+provider-specific fields or semantics.
+"""
+
 from dataclasses import dataclass
 from datetime import datetime
 from typing import Optional, Dict
@@ -8,19 +19,37 @@ class MailIntakeMessage:
     """
     Canonical internal representation of a single email message.
 
-    This model is provider-agnostic and safe to persist.
-    No Gmail-specific fields should appear here.
+    This model represents a fully parsed and normalized email message.
+    It is intentionally provider-agnostic and suitable for persistence,
+    indexing, and downstream processing.
+
+    No provider-specific identifiers, payloads, or API semantics
+    should appear in this model.
     """
 
     message_id: str
+    """Provider-specific message identifier."""
+
     thread_id: str
+    """Provider-specific thread identifier to which this message belongs."""
+
     timestamp: datetime
+    """Message timestamp as a timezone-naive UTC datetime."""
 
     from_email: str
+    """Sender email address."""
+
     from_name: Optional[str]
+    """Optional human-readable sender name."""
 
     subject: str
+    """Raw subject line of the message."""
+
     body_text: str
+    """Extracted plain-text body content of the message."""
+
     snippet: str
+    """Short provider-supplied preview snippet of the message."""
 
     raw_headers: Dict[str, str]
+    """Normalized mapping of message headers (header name → value)."""
diff --git a/mail_intake/models/thread.py b/mail_intake/models/thread.py
index ddcf757..6cd85c3 100644
--- a/mail_intake/models/thread.py
+++ b/mail_intake/models/thread.py
@@ -1,3 +1,13 @@
+"""
+Thread domain models for Mail Intake.
+
+This module defines the **canonical, provider-agnostic representation**
+of an email thread as used internally by the Mail Intake ingestion pipeline.
+
+Threads group related messages and serve as the primary unit of reasoning
+for higher-level correspondence workflows.
+"""
+
 from dataclasses import dataclass, field
 from datetime import datetime
 from typing import List, Set
@@ -10,21 +20,40 @@ class MailIntakeThread:
     """
     Canonical internal representation of an email thread.
 
-    Threads are the primary unit of reasoning for correspondence
-    workflows (job applications, interviews, follow-ups, etc.).
+    A thread groups multiple related messages under a single subject
+    and participant set. It is designed to support reasoning over
+    conversational context such as job applications, interviews,
+    follow-ups, and ongoing discussions.
+
+    This model is provider-agnostic and safe to persist.
     """
 
     thread_id: str
+    """Provider-specific thread identifier."""
+
     normalized_subject: str
+    """Normalized subject line used to group related messages."""
 
     participants: Set[str] = field(default_factory=set)
+    """Set of unique participant email addresses observed in the thread."""
+
     messages: List[MailIntakeMessage] = field(default_factory=list)
+    """Ordered list of messages belonging to this thread."""
 
     last_activity_at: datetime | None = None
+    """Timestamp of the most recent message in the thread."""
 
     def add_message(self, message: MailIntakeMessage) -> None:
         """
         Add a message to the thread and update derived fields.
+
+        This method:
+        - Appends the message to the thread
+        - Tracks unique participants
+        - Updates the last activity timestamp
+
+        Args:
+            message: Parsed mail message to add to the thread.
         """
         self.messages.append(message)
 
diff --git a/mail_intake/parsers/__init__.py b/mail_intake/parsers/__init__.py
index e69de29..f2d4855 100644
--- a/mail_intake/parsers/__init__.py
+++ b/mail_intake/parsers/__init__.py
@@ -0,0 +1,30 @@
+"""
+Message parsing utilities for Mail Intake.
+
+This package contains **provider-aware but adapter-agnostic parsing helpers**
+used to extract and normalize structured information from raw mail payloads.
+
+Parsers in this package are responsible for:
+- Interpreting provider-native message structures
+- Extracting meaningful fields such as headers, body text, and subjects
+- Normalizing data into consistent internal representations
+
+This package does not:
+- Perform network or IO operations
+- Contain provider API logic
+- Construct domain models directly
+
+Parsing functions are designed to be composable and are orchestrated by the
+ingestion layer.
+"""
+
+from .body import extract_body
+from .headers import parse_headers, extract_sender
+from .subject import normalize_subject
+
+__all__ = [
+    "extract_body",
+    "parse_headers",
+    "extract_sender",
+    "normalize_subject",
+]
diff --git a/mail_intake/parsers/body.py b/mail_intake/parsers/body.py
index 82e14de..2ef4877 100644
--- a/mail_intake/parsers/body.py
+++ b/mail_intake/parsers/body.py
@@ -1,3 +1,13 @@
+"""
+Message body extraction utilities for Mail Intake.
+
+This module contains helper functions for extracting a best-effort
+plain-text body from provider-native message payloads.
+
+The logic is intentionally tolerant of malformed or partial data and
+prefers human-readable text over fidelity to original formatting.
+"""
+
 import base64
 from typing import Dict, Any, Optional
 
@@ -9,6 +19,18 @@ from mail_intake.exceptions import MailIntakeParsingError
 def _decode_base64(data: str) -> str:
     """
     Decode Gmail URL-safe base64 payload into UTF-8 text.
+
+    Gmail message bodies are encoded using URL-safe base64, which may
+    omit padding and use non-standard characters.
+
+    Args:
+        data: URL-safe base64-encoded string.
+
+    Returns:
+        Decoded UTF-8 text with replacement for invalid characters.
+
+    Raises:
+        MailIntakeParsingError: If decoding fails.
     """
     try:
         padded = data.replace("-", "+").replace("_", "/")
@@ -21,6 +43,16 @@ def _decode_base64(data: str) -> str:
 def _extract_from_part(part: Dict[str, Any]) -> Optional[str]:
     """
     Extract text content from a single MIME part.
+
+    Supports:
+    - text/plain
+    - text/html (converted to plain text)
+
+    Args:
+        part: MIME part dictionary from a provider payload.
+
+    Returns:
+        Extracted plain-text content, or None if unsupported or empty.
     """
     mime_type = part.get("mimeType")
     body = part.get("body", {})
@@ -49,7 +81,14 @@ def extract_body(payload: Dict[str, Any]) -> str:
     Priority:
     1. text/plain
     2. text/html (stripped to text)
-    3. empty string (if nothing usable found)
+    3. Single-part body
+    4. empty string (if nothing usable found)
+
+    Args:
+        payload: Provider-native message payload dictionary.
+
+    Returns:
+        Extracted plain-text message body.
     """
     if not payload:
         return ""
diff --git a/mail_intake/parsers/headers.py b/mail_intake/parsers/headers.py
index 7c0c929..74fdc87 100644
--- a/mail_intake/parsers/headers.py
+++ b/mail_intake/parsers/headers.py
@@ -1,3 +1,13 @@
+"""
+Message header parsing utilities for Mail Intake.
+
+This module provides helper functions for normalizing and extracting
+useful information from provider-native message headers.
+
+The functions here are intentionally simple and tolerant of malformed
+or incomplete header data.
+"""
+
 from typing import Dict, List, Tuple, Optional
 
 
@@ -5,19 +15,29 @@ def parse_headers(raw_headers: List[Dict[str, str]]) -> Dict[str, str]:
     """
     Convert a list of Gmail-style headers into a normalized dict.
 
-    Input:
-    [
-        {"name": "From", "value": "John Doe <john@example.com>"},
-        {"name": "Subject", "value": "Re: Interview Update"},
-        ...
-    ]
+    Provider payloads (such as Gmail) typically represent headers as a list
+    of name/value mappings. This function normalizes them into a
+    case-insensitive dictionary keyed by lowercase header names.
 
-    Output:
-    {
-        "from": "...",
-        "subject": "...",
-        ...
-    }
+    Args:
+        raw_headers: List of header dictionaries, each containing
+            ``name`` and ``value`` keys.
+
+    Returns:
+        Dictionary mapping lowercase header names to stripped values.
+
+    Example:
+        Input:
+            [
+                {"name": "From", "value": "John Doe <john@example.com>"},
+                {"name": "Subject", "value": "Re: Interview Update"},
+            ]
+
+        Output:
+            {
+                "from": "John Doe <john@example.com>",
+                "subject": "Re: Interview Update",
+            }
     """
     headers: Dict[str, str] = {}
 
@@ -37,18 +57,27 @@ def extract_sender(headers: Dict[str, str]) -> Tuple[str, Optional[str]]:
     """
     Extract sender email and optional display name from headers.
 
-    Returns:
-        (email, name)
+    This function parses the ``From`` header and attempts to extract:
+    - Sender email address
+    - Optional human-readable display name
 
-    If name cannot be determined, name will be None.
+    Args:
+        headers: Normalized header dictionary as returned by
+            :func:`parse_headers`.
+
+    Returns:
+        A tuple ``(email, name)`` where:
+        - ``email`` is the sender email address
+        - ``name`` is the display name, or ``None`` if unavailable
+
+    Examples:
+        ``"John Doe <john@example.com>"`` → ``("john@example.com", "John Doe")``
+        ``"john@example.com"`` → ``("john@example.com", None)``
     """
     from_header = headers.get("from")
     if not from_header:
         return "", None
 
-    # Common forms:
-    #   Name <email@domain>
-    #   email@domain
     if "<" in from_header and ">" in from_header:
         name_part, email_part = from_header.split("<", 1)
         email = email_part.rstrip(">").strip()
diff --git a/mail_intake/parsers/subject.py b/mail_intake/parsers/subject.py
index df5041d..960a71f 100644
--- a/mail_intake/parsers/subject.py
+++ b/mail_intake/parsers/subject.py
@@ -1,7 +1,18 @@
+"""
+Subject line normalization utilities for Mail Intake.
+
+This module provides helper functions for normalizing email subject lines
+to enable reliable thread-level comparison and grouping.
+
+Normalization is intentionally conservative to avoid altering semantic
+meaning while removing common reply and forward prefixes.
+"""
+
 import re
 
 
 _PREFIX_RE = re.compile(r"^(re|fw|fwd)\s*:\s*", re.IGNORECASE)
+"""Regular expression matching common reply/forward subject prefixes."""
 
 
 def normalize_subject(subject: str) -> str:
@@ -9,11 +20,19 @@ def normalize_subject(subject: str) -> str:
     Normalize an email subject for thread-level comparison.
 
     Operations:
-    - Strip common prefixes (Re:, Fwd:, FW:)
-    - Collapse whitespace
-    - Preserve original casing (no lowercasing)
+    - Strips common prefixes such as ``Re:``, ``Fwd:``, and ``FW:``
+    - Repeats prefix stripping to handle stacked prefixes
+    - Collapses excessive whitespace
+    - Preserves original casing (no lowercasing)
 
-    This function is intentionally conservative.
+    This function is intentionally conservative and avoids aggressive
+    transformations that could alter the semantic meaning of the subject.
+
+    Args:
+        subject: Raw subject line from a message header.
+
+    Returns:
+        Normalized subject string suitable for thread grouping.
     """
     if not subject:
         return ""