diff --git a/src/lib.rs b/src/lib.rs
index 7181455..ac9959e 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -77,6 +77,12 @@
 //! as a reminder that you are outside of the normal semver guarantees.
 //!
 //! Semver exempt methods are marked as such in the proc-macro2 documentation.
+//!
+//! # Thread-Safety
+//!
+//! Most types in this crate are `!Sync` because the underlying compiler
+//! types make use of thread-local memory, meaning they cannot be accessed from
+//! a different thread.
 
 // Proc-macro2 types in rustdoc of other crates get linked to here.
 #![doc(html_root_url = "https://docs.rs/proc-macro2/0.4.30")]