There are several free Common Lisp implementations with which Femlisp is known to work completely, namely CMUCL and SBCL. It runs also on the commercial implementations Scieneer CL, Allegro CL, and Lispworks. Femlisp should also run with ECL and Clozure CL, however, it is not regularly tested with those implementations. Similarly, using Femlisp with CLISP works, but does not make much sense, but is very slow since CLISP does not compile to native code.