diff --git a/tzk/__main__.py b/tzk/__main__.py
index d763b36..fecb303 100644
--- a/tzk/__main__.py
+++ b/tzk/__main__.py
@@ -13,15 +13,26 @@ from tzk.util import BuildError, fail, numerize
 
 
 class CliCommand(ABC):
+    """
+    Base class for subcommands of tzk.
+    """
+    #: The text of the subcommand to be used on the command line.
     cmd = None   # type: str
+    #: Help string for argparse to display for this subcommand.
     help = None  # type: str
 
     @abstractclassmethod
     def setup_arguments(cls, parser: argparse.ArgumentParser) -> None:
+        """
+        Given the :arg:`parser`, add any arguments this subcommand wants to accept.
+        """
         raise NotImplementedError
 
     @abstractmethod
-    def execute(self, parser: argparse.Namespace) -> None:
+    def execute(self, args: argparse.Namespace) -> None:
+        """
+        Given the :arg:`args` passed to this subcommand, do whatever the command does.
+        """
         raise NotImplementedError
 
 
@@ -182,21 +193,27 @@ class BuildCommand(CliCommand):
     def execute(self, args: argparse.Namespace) -> None:
         self._precheck(args.product)
 
+        # Find the build steps for the product the user specified.
         steps = cm().products[args.product]
         print(f"tzk: Starting build of product '{args.product}'.")
         print(f"tzk: Found {len(steps)} build {numerize(len(steps), 'step')}.")
 
+        # For each build step...
         for idx, step in enumerate(steps, 1):
+            # Explain what we're doing. Use first line of the builder's docstring
+            # as a summary, if present.
             if hasattr(step, '__doc__'):
                 short_description = step.__doc__.strip().split('\n')[0].rstrip('.')
                 print(f"tzk: Step {idx}/{len(steps)}: {short_description}")
             else:
                 print(f"tzk: Step {idx}/{len(steps)}")
 
+            # If the user asked to skip this builder on the command line, do so.
             if step.__name__ in args.skip_builder:
                 print(f"tzk: Skipping step {idx} due to --skip-builder parameter.")
                 continue
 
+            # Execute step and handle any errors.
             try:
                 step()
             except BuildError as e:
@@ -231,10 +248,12 @@ def launch():
 
     args = parser.parse_args()
     if not hasattr(args, '_cls'):
+        # no subcommand was given
         parser.print_help()
         sys.exit(0)
 
     # For all operations except 'init', we start in the wiki folder.
+    # (For init, we're actually *creating* the wiki folder.)
     if not args._cls.cmd == "init":
         if not cm().wiki_folder:
             fail("No 'wiki_folder' option found in config. Set this option to the name "
@@ -256,4 +275,4 @@ def launch():
 
 
 if __name__ == '__main__':
-    launch()
\ No newline at end of file
+    launch()
diff --git a/tzk/config.py b/tzk/config.py
index 9b78a79..eccb5ae 100644
--- a/tzk/config.py
+++ b/tzk/config.py
@@ -1,3 +1,6 @@
+"""
+config.py - read and manage the TZK config file
+"""
 import datetime
 import functools
 import importlib
@@ -57,6 +60,14 @@ class ConfigurationManager:
 
 
 def cm(cache=[]):
+    """
+    Call this function to retrieve the singleton ConfigurationManager object,
+    reading and initializing it if necessary.
+
+    Since so much happens when the ConfigurationManager is initialized,
+    this has to go in a function so that autodoc doesn't blow up
+    when it tries to import the module.
+    """
     if not cache:
         cache.append(ConfigurationManager())
     return cache[0]
diff --git a/tzk/git.py b/tzk/git.py
index 6cb4f73..d9d4566 100644
--- a/tzk/git.py
+++ b/tzk/git.py
@@ -1,11 +1,17 @@
 import subprocess
 from typing import Sequence
 
-def exec(*args: str):
-    return subprocess.check_call(["git", *args])
 
-def rc(*args: str):
+def exec(*args: str) -> None:
+    "Execute a Git command, raising CalledProcessError if the exit code is nonzero."
+    subprocess.check_call(["git", *args])
+
+
+def rc(*args: str) -> int:
+    "Execute a Git command, returning the exit code."
     return subprocess.call(["git", *args])
 
-def read(*args: str):
+
+def read(*args: str) -> str:
+    "Execute a Git command, returning the output as a string."
     return subprocess.check_output(["git", *args], text=True).strip()
diff --git a/tzk/tw.py b/tzk/tw.py
index 0aa989d..0c8b333 100644
--- a/tzk/tw.py
+++ b/tzk/tw.py
@@ -20,6 +20,7 @@ def _tw_path() -> str:
 
 @functools.lru_cache(1)
 def _whoami() -> str:
+    "Try to guess the user's name."
     try:
         return subprocess.check_output(("whoami",), text=True).strip()
     except subprocess.CalledProcessError:
@@ -27,6 +28,19 @@ def _whoami() -> str:
 
 
 def exec(args: Sequence[Sequence[str]], base_wiki_folder: str = None) -> int:
+    """
+    Execute a series of TiddlyWiki commands.
+
+    :param args: A list of lists of CLI commands to send to TiddlyWiki.
+                 The first element of each list is a TiddlyWiki CLI command,
+                 without the ``--``, e.g., ``savewikifolder``.
+                 The following elements of the list are arguments to that command.
+    :param base_wiki_folder: If the wiki to execute commands against is not the one
+                             in the current directory, provide its path here.
+                             The current directory is the source wiki's root directory
+                             during the execution of builders,
+                             unless explicitly changed.
+    """
     call_args = [_tw_path()]
     if base_wiki_folder is not None:
         call_args.append(base_wiki_folder)
@@ -98,6 +112,10 @@ def _save_wikifolder_to_config(wiki_name: str) -> bool:
 
 
 def _add_filesystem_plugins(wiki_name: str) -> None:
+    """
+    Add the "tiddlywiki/filesystem" and "tiddlywiki/tiddlyweb" plugins
+    required for Node.js client-server operation to the new wiki's tiddlywiki.info.
+    """
     print("tzk: Adding filesystem plugins to tiddlywiki.info...")
     info_path = Path.cwd() / wiki_name / "tiddlywiki.info"
     with info_path.open("r") as f:
@@ -108,6 +126,9 @@ def _add_filesystem_plugins(wiki_name: str) -> None:
 
 
 def _init_gitignore() -> None:
+    """
+    Create a basic gitignore for the new wiki.
+    """
     print("tzk: Creating gitignore...")
     GITIGNORE = dedent("""
     __pycache__/
@@ -122,6 +143,9 @@ def _init_gitignore() -> None:
 
 
 def _initial_commit() -> None:
+    """
+    Create a new Git repo and commit everything we've done so far.
+    """
     print("tzk: Initializing new Git repository for wiki...")
     git.exec("init")
 
@@ -131,6 +155,9 @@ def _initial_commit() -> None:
 
 
 def install(wiki_name: str, tw_version_spec: str, author: Optional[str]):
+    """
+    Install TiddlyWiki on Node.js in the current directory and set up a new wiki.
+    """
     # assert: caller has checked npm and git are installed
     warnings = False
 
diff --git a/tzk/util.py b/tzk/util.py
index 3962f92..8d170ad 100644
--- a/tzk/util.py
+++ b/tzk/util.py
@@ -1,3 +1,6 @@
+"""
+util.py - miscellaneous utility functions
+"""
 from contextlib import contextmanager
 import os
 import sys
@@ -9,11 +12,13 @@ class BuildError(Exception):
 
 
 def fail(msg: str, exit_code: int = 1) -> NoReturn:
+    "Print message to stderr and quit with exit code 1."
     print(msg, file=sys.stderr)
     sys.exit(exit_code)
 
 
-def numerize(number: int, singular: str, plural: str = None):
+def numerize(number: int, singular: str, plural: str = None) -> str:
+    "Render a string in the singular or plural as appropriate."
     if plural is None:
         plural = singular + 's'