diff --git a/src/tools/timer.rs b/src/tools/timer.rs
index 8130dcce..a1910f2f 100644
--- a/src/tools/timer.rs
+++ b/src/tools/timer.rs
@@ -1,3 +1,8 @@
+//! POSIX per-process timer interface.
+//!
+//! This module provides a wrapper around POSIX timers (see `timer_create(2)`) and utilities to
+//! setup thread-targeted signaling and signal masks.
+
 use std::{io, mem};
 use std::time::Duration;