An Introduction To Pthreads-Tcl - Document describing the changes to Tcl that enable it to be used in conjunction with POSIX Threads (pthreads).
Make a Tcl Extension Thread-Safe - The basics of making an extension thread-safe are very simple, if a bit crude. This article shows you how and why.
Tcl Threading Model - This brief document describes the model Tcl uses for threads, which is that one thread can have one or more interpreters, but each interpreter is bound to a particular thread.