diff --git a/src/lib/arch/lib_arch.dox b/src/lib/arch/lib_arch.dox
index 60b5fafeb4..f7cfdbf0e3 100644
--- a/src/lib/arch/lib_arch.dox
+++ b/src/lib/arch/lib_arch.dox
@@ -1,4 +1,4 @@
 /**
 @dir lib/arch
-@brief lib/arch
+@brief lib/arch: Compatibility code for handling different CPU architectures.
 **/
diff --git a/src/lib/cc/lib_cc.dox b/src/lib/cc/lib_cc.dox
index 804260cb29..880cfc44fb 100644
--- a/src/lib/cc/lib_cc.dox
+++ b/src/lib/cc/lib_cc.dox
@@ -1,4 +1,4 @@
 /**
 @dir lib/cc
-@brief lib/cc
+@brief lib/cc: Macros for managing the C compiler and language.
 **/
diff --git a/src/lib/conf/lib_conf.dox b/src/lib/conf/lib_conf.dox
index 40a1d9f90f..2ad051dcf2 100644
--- a/src/lib/conf/lib_conf.dox
+++ b/src/lib/conf/lib_conf.dox
@@ -1,4 +1,5 @@
 /**
 @dir lib/conf
-@brief lib/conf
+@brief lib/conf: Types and macros for declaring configuration options.
+
 **/
diff --git a/src/lib/container/lib_container.dox b/src/lib/container/lib_container.dox
index 6ee719f47e..9599cce6ea 100644
--- a/src/lib/container/lib_container.dox
+++ b/src/lib/container/lib_container.dox
@@ -1,4 +1,5 @@
 /**
 @dir lib/container
-@brief lib/container
+@brief lib/container: Hash tables, dynamic arrays, bit arrays, etc.
+
 **/
diff --git a/src/lib/ctime/lib_ctime.dox b/src/lib/ctime/lib_ctime.dox
index 476c95991c..bf95b03165 100644
--- a/src/lib/ctime/lib_ctime.dox
+++ b/src/lib/ctime/lib_ctime.dox
@@ -1,4 +1,16 @@
 /**
 @dir lib/ctime
-@brief lib/ctime
+@brief lib/ctime: Constant-time code to avoid side-channels.
+
+This module contains constant-time implementations of various
+data comparison and table lookup functions.  We use these in preference to
+memcmp() and so forth, since memcmp() can leak information about its inputs
+based on how fast it returns.  In general, your code should call tor_memeq()
+and tor_memneq(), not memcmp().
+
+We also define some _non_-constant-time wrappers for memcmp() here: Since we
+consider calls to memcmp() to be in error, we require that code that actually
+doesn't need to be constant-time to use the fast_memeq() / fast_memneq() /
+fast_memcmp() aliases instead.
+
 **/
diff --git a/src/lib/defs/lib_defs.dox b/src/lib/defs/lib_defs.dox
index 5adb527fc7..cd39414bfe 100644
--- a/src/lib/defs/lib_defs.dox
+++ b/src/lib/defs/lib_defs.dox
@@ -1,4 +1,4 @@
 /**
 @dir lib/defs
-@brief lib/defs
+@brief lib/defs: Lowest-level constants, used in many places.
 **/
diff --git a/src/lib/dispatch/lib_dispatch.dox b/src/lib/dispatch/lib_dispatch.dox
index f194eff481..9f3a1fc7df 100644
--- a/src/lib/dispatch/lib_dispatch.dox
+++ b/src/lib/dispatch/lib_dispatch.dox
@@ -1,4 +1,16 @@
 /**
 @dir lib/dispatch
-@brief lib/dispatch
+@brief lib/dispatch: In-process message delivery.
+
+This module provides a general in-process "message dispatch" system in which
+typed messages are sent on channels. The dispatch.h header has far more
+information.
+
+It is used by by \ref src/lib/pubsub "lib/pubsub" to implement our general
+inter-module publish/subscribe system.
+
+This is not a fancy multi-threaded many-to-many dispatcher as you may be used
+to from more sophisticated architectures: this dispatcher is intended only
+for use in improving Tor's architecture.
+
 **/
diff --git a/src/lib/encoding/lib_encoding.dox b/src/lib/encoding/lib_encoding.dox
index 4a5fad9271..19aca645fb 100644
--- a/src/lib/encoding/lib_encoding.dox
+++ b/src/lib/encoding/lib_encoding.dox
@@ -1,4 +1,8 @@
 /**
 @dir lib/encoding
-@brief lib/encoding
+@brief lib/encoding: Encoding data in various forms, types, and transformations
+
+Here we have time formats (timefmt.c), quoted strings (qstring.c), C strings
+(string.c) base-16/32/64 (binascii.c), and more.
+
 **/
diff --git a/src/lib/err/lib_err.dox b/src/lib/err/lib_err.dox
index 8994fa5fd8..23f9d9d3a7 100644
--- a/src/lib/err/lib_err.dox
+++ b/src/lib/err/lib_err.dox
@@ -1,4 +1,15 @@
 /**
 @dir lib/err
-@brief lib/err
+@brief lib/err: Lowest-level error handling code.
+
+This module is responsible for generating stack traces, handling raw
+assertion failures, and otherwise reporting problems that might not be
+safe to report via the regular logging module.
+
+There are three kinds of users for the functions in this module:
+  * Code that needs a way to assert(), but which cannot use the regular
+    `tor_assert()` macros in logging module.
+  * Code that needs signal-safe error reporting.
+  * Higher-level error handling code.
+
 **/
diff --git a/src/lib/fdio/lib_fdio.dox b/src/lib/fdio/lib_fdio.dox
index b868d28aab..c3ca3b9c88 100644
--- a/src/lib/fdio/lib_fdio.dox
+++ b/src/lib/fdio/lib_fdio.dox
@@ -1,4 +1,7 @@
 /**
 @dir lib/fdio
-@brief lib/fdio
+@brief lib/fdio Code to read/write on file descriptors.
+
+(This module also handles sockets, on platforms where a socket is not a kind
+of fd.)
 **/
diff --git a/src/lib/intmath/lib_intmath.dox b/src/lib/intmath/lib_intmath.dox
index ce71e455d1..0d5d711eea 100644
--- a/src/lib/intmath/lib_intmath.dox
+++ b/src/lib/intmath/lib_intmath.dox
@@ -1,4 +1,4 @@
 /**
 @dir lib/intmath
-@brief lib/intmath
+@brief lib/intmath Integer mathematics.
 **/
diff --git a/src/lib/lib.dox b/src/lib/lib.dox
index f1b2291c76..1d3bc3a3fe 100644
--- a/src/lib/lib.dox
+++ b/src/lib/lib.dox
@@ -1,8 +1,151 @@
 /**
-@dir lib
+@dir /lib
 @brief lib: low-level functionality.
 
-The "lib" directory contains low-level functionality, most of it not
-necessarily Tor-specific.
+The "lib" directory contains low-level functionality.  In general, this
+code is not necessarily Tor-specific, but is instead possibly useful for
+other applications.
+
+The modules in `lib` are currently well-factored: each one depends
+only on lower-level modules.  You can see an up-to-date list of the
+modules, sorted from lowest to highest level, by running
+`./scripts/maint/practracker/includes.py --toposort`.
+
+As of this writing, the library modules are (from lowest to highest
+level):
+
+   - \ref src/lib/cc "lib/cc" -- Macros for managing the C compiler and
+     language.
+
+   - \ref src/lib/version "lib/version" -- Holds the current version of Tor.
+
+   - \ref src/lib/testsupport "lib/testsupport" -- Helpers for making
+     test-only code, and test mocking support.
+
+   - \ref src/lib/defs "lib/defs" -- Lowest-level constants.
+
+   - \ref src/lib/subsys "lib/subsys" -- Types used for declaring a
+     "subsystem". (_A subsystem is a module with support for initialization,
+     shutdown, configuration, and so on._)
+
+   - \ref src/lib/conf "lib/conf" -- For declaring configuration options.
+
+   - \ref src/lib/arch "lib/arch" -- For handling differences in CPU
+     architecture.
+
+   - \ref src/lib/err "lib/err" -- Lowest-level error handling code.
+
+   - \ref src/lib/malloc "lib/malloc" -- Memory management.
+     management.
+
+   - \ref src/lib/intmath "lib/intmath" -- Integer mathematics.
+
+   - \ref src/lib/fdio "lib/fdio" -- For
+      reading and writing n file descriptors.
+
+   - \ref src/lib/lock "lib/lock" -- Simple locking support.
+      (_Lower-level than the rest of the threading code._)
+
+   - \ref src/lib/ctime "lib/ctime" -- Constant-time code to avoid
+     side-channels.
+
+   - \ref src/lib/string "lib/string" -- Low-level string manipulation.
+
+   - \ref src/lib/wallclock "lib/wallclock" --
+     For inspecting and manipulating the current (UTC) time.
+
+   - \ref src/lib/osinfo "lib/osinfo" -- For inspecting the OS version
+     and capabilities.
+
+   - \ref src/lib/smartlist_core "lib/smartlist_core" -- The bare-bones
+     pieces of our dynamic array ("smartlist") implementation.
+
+   - \ref src/lib/log "lib/log" -- Log messages to files, syslogs, etc.
+
+   - \ref src/lib/container "lib/container" -- General purpose containers,
+     including dynamic arrays ("smartlists"), hashtables, bit arrays,
+     etc.
+
+   - \ref src/lib/trace "lib/trace" -- A general-purpose API
+     function-tracing functionality Tor.  (_Currently not much used._)
+
+   - \ref src/lib/thread "lib/thread" -- Threading compatibility and utility
+     functionality, other than low-level locks (which are in \ref
+     src/lib/lock "lib/lock") and workqueue/threadpool code (which belongs
+     in \ref src/lib/evloop "lib/evloop").
+
+   - \ref src/lib/term "lib/term" -- Terminal manipulation
+     (like reading a password from the user).
+
+   - \ref src/lib/memarea "lib/memarea" -- A fast
+     "arena" style allocator, where the data is freed all at once.
+
+   - \ref src/lib/encoding "lib/encoding" -- Encoding
+     data in various formats, datatypes, and transformations.
+
+   - \ref src/lib/dispatch "lib/dispatch" -- A general-purpose in-process
+     message delivery system.
+
+   - \ref src/lib/sandbox "lib/sandbox" -- Our Linux seccomp2 sandbox
+     implementation.
+
+   - \ref src/lib/pubsub "lib/pubsub" -- Code and macros to implement our
+     publish/subscribe message passing system.
+
+   - \ref src/lib/fs "lib/fs" -- Utility and compatibility code for
+     manipulating files, filenames, directories, and so on.
+
+   - \ref src/lib/confmgt "lib/confmgt" -- Code to parse, encode, and
+     manipulate our configuration files, state files, and so forth.
+
+   - \ref src/lib/crypt_ops "lib/crypt_ops" -- Cryptographic operations. This
+     module contains wrappers around the cryptographic libraries that we
+     support, and implementations for some higher-level cryptographic
+     constructions that we use.
+
+   - \ref src/lib/meminfo "lib/meminfo" -- Functions for inspecting our
+     memory usage, if the malloc implementation exposes that to us.
+
+   - \ref src/lib/time "lib/time" -- Higher level time functions, including
+     fine-gained and monotonic timers.
+
+   - \ref src/lib/math "lib/math" -- Floating-point mathematical utilities,
+     including compatibility code, and probability distributions.
+
+   - \ref src/lib/buf "lib/buf" -- A general purpose queued buffer
+     implementation, similar to the BSD kernel's "mbuf" structure.
+
+   - \ref src/lib/net "lib/net" -- Networking code, including address
+     manipulation, compatibility wrappers,
+
+   - \ref src/lib/compress "lib/compress" -- A compatibility wrapper around
+     several compression libraries, currently including zlib, zstd, and lzma.
+
+   - \ref src/lib/geoip "lib/geoip" -- Utilities to manage geoip (IP to
+      country) lookups and formats.
+
+   - \ref src/lib/tls "lib/tls" -- Compatibility wrappers around the library
+     (NSS or OpenSSL, depending on configuration) that Tor uses to implement
+     the TLS link security protocol.
+
+   - \ref src/lib/evloop "lib/evloop" -- Tools to manage the event loop and
+     related functionality, in order to implement asynchronous networking,
+     timers, periodic events, and other scheduling tasks.
+
+   - \ref src/lib/process "lib/process" -- Utilities and compatibility code
+     to launch and manage subprocesses.
+
+### What belongs in lib?
+
+In general, if you can imagine some program wanting the functionality
+you're writing, even if that program had nothing to do with Tor, your
+functionality belongs in lib.
+
+If it falls into one of the existing "lib" categories, your
+functionality belongs in lib.
+
+If you are using platform-specific `ifdef`s to manage compatibility
+issues among platforms, you should probably consider whether you can
+put your code into lib.
 
 **/
diff --git a/src/lib/lock/lib_lock.dox b/src/lib/lock/lib_lock.dox
index 44693e7a69..f248e12c86 100644
--- a/src/lib/lock/lib_lock.dox
+++ b/src/lib/lock/lib_lock.dox
@@ -1,4 +1,8 @@
 /**
 @dir lib/lock
-@brief lib/lock
+@brief lib/lock: Simple locking support.
+
+This module is more low-level than the rest of the threading code, since it
+is needed by more intermediate-level modules.
+
 **/
diff --git a/src/lib/log/lib_log.dox b/src/lib/log/lib_log.dox
index 915d652407..5059f3ef73 100644
--- a/src/lib/log/lib_log.dox
+++ b/src/lib/log/lib_log.dox
@@ -1,4 +1,12 @@
 /**
 @dir lib/log
-@brief lib/log
+@brief lib/log: Log messages to files, syslogs, etc.
+
+You can think of this as the logical "midpoint" of the
+\ref src/lib "library code": much of the higher-level code is higher-level
+_because_ it uses the logging module, and much of the lower-level code is
+specifically written to avoid having to log, because the logging module
+depends on it.
+
+
 **/
diff --git a/src/lib/malloc/lib_malloc.dox b/src/lib/malloc/lib_malloc.dox
index 4923f14463..6408d7ce30 100644
--- a/src/lib/malloc/lib_malloc.dox
+++ b/src/lib/malloc/lib_malloc.dox
@@ -1,4 +1,5 @@
 /**
 @dir lib/malloc
-@brief lib/malloc
+@brief lib/malloc: Wrappers and utilities for memory management.
+
 **/
diff --git a/src/lib/memarea/lib_memarea.dox b/src/lib/memarea/lib_memarea.dox
index dbd98de5ec..f5b44f97c2 100644
--- a/src/lib/memarea/lib_memarea.dox
+++ b/src/lib/memarea/lib_memarea.dox
@@ -1,4 +1,10 @@
 /**
 @dir lib/memarea
-@brief lib/memarea
+@brief lib/memarea A fast arena-style allocator.
+
+This module has a fast "arena" style allocator, where memory is freed all at
+once.  This kind of allocation is very fast and avoids fragmentation, at the
+expense of requiring all the data to be freed at the same time.  We use this
+for parsing and diff calculations.
+
 **/
diff --git a/src/lib/osinfo/lib_osinfo.dox b/src/lib/osinfo/lib_osinfo.dox
index 7733755f20..c78615937b 100644
--- a/src/lib/osinfo/lib_osinfo.dox
+++ b/src/lib/osinfo/lib_osinfo.dox
@@ -1,4 +1,10 @@
 /**
 @dir lib/osinfo
-@brief lib/osinfo
+@brief lib/osinfo: For inspecting the OS version and capabilities.
+
+In general, we use this module when we're telling the user what operating
+system they are running.  We shouldn't make decisions based on the output of
+these checks: instead, we should have more specific checks, either at compile
+time or run time, based on the observed system behavior.
+
 **/
diff --git a/src/lib/sandbox/lib_sandbox.dox b/src/lib/sandbox/lib_sandbox.dox
index eb42d97589..8b33fc2553 100644
--- a/src/lib/sandbox/lib_sandbox.dox
+++ b/src/lib/sandbox/lib_sandbox.dox
@@ -1,4 +1,17 @@
 /**
 @dir lib/sandbox
-@brief lib/sandbox
+@brief lib/sandbox Linux seccomp2-based sandbox.
+
+This module uses Linux's seccomp2 facility via the
+[`libseccomp` library](https://github.com/seccomp/libseccomp), to restrict
+the set of system calls that Tor is allowed to invoke while it is running.
+
+Because there are many libc versions that invoke different system calls, and
+because handling strings is quite complex, this module is more complex and
+less portable than it needs to be.
+
+A better architecture would put the responsibility for invoking tricky system
+calls (like open()) in another, less restricted process, and give that
+process responsibility for enforcing our sandbox rules.
+
 **/
diff --git a/src/lib/smartlist_core/lib_smartlist_core.dox b/src/lib/smartlist_core/lib_smartlist_core.dox
index 507d0fe92f..68b8249351 100644
--- a/src/lib/smartlist_core/lib_smartlist_core.dox
+++ b/src/lib/smartlist_core/lib_smartlist_core.dox
@@ -1,4 +1,12 @@
 /**
 @dir lib/smartlist_core
-@brief lib/smartlist_core
+@brief lib/smartlist_core: Minimal dynamic array implementation
+
+A `smartlist_t` is a dynamic array type for holding `void *`.  We use it
+throughout the rest of the codebase.
+
+There are higher-level pieces in \ref src/lib/container "lib/container", but
+the ones in lib/smartlist_core are used by the logging code, and therefore
+cannot use the logging code.
+
 **/
diff --git a/src/lib/string/lib_string.dox b/src/lib/string/lib_string.dox
index 3e038ea072..95645ab368 100644
--- a/src/lib/string/lib_string.dox
+++ b/src/lib/string/lib_string.dox
@@ -1,4 +1,15 @@
 /**
 @dir lib/string
-@brief lib/string
+@brief lib/string: Low-level string manipulation.
+
+We have a number of compatibility functions here: some are for handling
+functionality that is not implemented (or not implemented the same) on every
+platform; some are for providing locale-independent versions of libc
+functions that would otherwise be defined differently for different users.
+
+Other functions here are for common string-manipulation operations that we do
+in the rest of the codebase.
+
+Any string function high-level enough to need logging belongs in a
+higher-level module.
 **/
diff --git a/src/lib/subsys/lib_subsys.dox b/src/lib/subsys/lib_subsys.dox
index f9cd5eeb81..4d98ff577a 100644
--- a/src/lib/subsys/lib_subsys.dox
+++ b/src/lib/subsys/lib_subsys.dox
@@ -1,4 +1,34 @@
 /**
 @dir lib/subsys
-@brief lib/subsys
+@brief lib/subsys: Types for declaring a "subsystem".
+
+## Subsystems in Tor
+
+A subsystem is a module with support for initialization, shutdown,
+configuration, and so on.
+
+Many parts of Tor can be initialized, cleaned up, and configured somewhat
+independently through a table-driven mechanism.  Each such part is called a
+"subsystem".
+
+To declare a subsystem, make a global `const` instance of the `subsys_fns_t`
+type, filling in the function pointer fields that you require with ones
+corresponding to your subsystem.  Any function pointers left as "NULL" will
+be a no-op.  Each system must have a name and a "level", which corresponds to
+the order in which it is initialized.  (See `app/main/subsystem_list.c` for a
+list of current subsystems and their levels.)
+
+Then, insert your subsystem in the list in `app/main/subsystem_list.c`.  It
+will need to occupy a position corresponding to its level.
+
+At this point, your subsystem will be handled like the others: it will get
+initialized at startup, torn down at exit, and so on.
+
+Historical note: Not all of Tor's code is currently handled as
+subsystems. As you work with older code, you may see some parts of the code
+that are initialized from `tor_init()` or `run_tor_main_loop()` or
+`tor_run_main()`; and torn down from `tor_cleanup()`.  We aim to migrate
+these to subsystems over time; please don't add any new code that follows
+this pattern.
+
 **/
diff --git a/src/lib/testsupport/lib_testsupport.dox b/src/lib/testsupport/lib_testsupport.dox
index 63ccc47d34..665b10e1f9 100644
--- a/src/lib/testsupport/lib_testsupport.dox
+++ b/src/lib/testsupport/lib_testsupport.dox
@@ -1,4 +1,4 @@
 /**
 @dir lib/testsupport
-@brief lib/testsupport
+@brief lib/testsupport: Helpers for test-only code and for function mocking.
 **/
diff --git a/src/lib/trace/lib_trace.dox b/src/lib/trace/lib_trace.dox
index a1ae256506..a8ac09d879 100644
--- a/src/lib/trace/lib_trace.dox
+++ b/src/lib/trace/lib_trace.dox
@@ -1,4 +1,8 @@
 /**
 @dir lib/trace
-@brief lib/trace
+@brief lib/trace: Function-tracing functionality API.
+
+This module is used for adding "trace" support (low-granularity function
+logging) to Tor.  Right now it doesn't have many users.
+
 **/
diff --git a/src/lib/version/lib_version.dox b/src/lib/version/lib_version.dox
index 213e1a1ae8..d3d2481ba0 100644
--- a/src/lib/version/lib_version.dox
+++ b/src/lib/version/lib_version.dox
@@ -1,4 +1,4 @@
 /**
 @dir lib/version
-@brief lib/version
+@brief lib/version: holds the current version of Tor.
 **/
diff --git a/src/lib/wallclock/lib_wallclock.dox b/src/lib/wallclock/lib_wallclock.dox
index 7bb2b075d1..a024425ea9 100644
--- a/src/lib/wallclock/lib_wallclock.dox
+++ b/src/lib/wallclock/lib_wallclock.dox
@@ -1,4 +1,12 @@
 /**
 @dir lib/wallclock
-@brief lib/wallclock
+@brief lib/wallclock: Inspect and manipulate the current time.
+
+This module handles our concept of "what time is it" or "what time does the
+world agree it is?"  Generally, if you want something derived from UTC, this
+is the module for you.
+
+For versions of the time that are more local, more monotonic, or more
+accurate, see \ref src/lib/time "lib/time".
+
 **/