diff --git a/README.md b/README.md
index d0ecf7a..87b51e3 100644
--- a/README.md
+++ b/README.md
@@ -120,6 +120,13 @@ Pass tests are considered to succeed if they compile successfully and have a
 
 That's the entire API.
 
+### Conditional compilation
+
+Trybuild sets `cfg(trybuild)` when compiling test cases. In a test case,
+use `#[cfg(trybuild)]` to include code in these compilations, or
+`#[cfg(not(trybuild))]` to exclude it. Trybuild does not set this cfg for
+the outer test harness that calls `TestCases`.
+
 <br>
 
 ## Workflow
diff --git a/src/lib.rs b/src/lib.rs
index c6d06d0..05ed62d 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -105,6 +105,13 @@
 //!
 //! That's the entire API.
 //!
+//! ## Conditional compilation
+//!
+//! Trybuild sets `cfg(trybuild)` when compiling test cases. In a test case,
+//! use `#[cfg(trybuild)]` to include code in these compilations, or
+//! `#[cfg(not(trybuild))]` to exclude it. Trybuild does not set this cfg for
+//! the outer test harness that calls `TestCases`.
+//!
 //! <br>
 //!
 //! # Workflow