Most Common Lisp implementations have had good threading support for years now, at least on the implementation's primary platforms. The threading APIs, however

Most Common Lisp implementations have had good threading support for years now, at least on the implementation's primary platforms. The threading APIs, however, are all different -- not necessarily in any deep way, but in incidental ways like function names and signatures. There's a package Bordeaux-Threads that provides a portable API layer on top of the various implementation-specific APIs.