And it's ridiculous to talk about what "lisp programs" do, since there have been hundreds of implementations of list over the last forty-five years, and they all have different garbage collectors. Saying that "lisp programs do this" or "lisp programs do that" is almost as meaningless as saying "computers do this" or "computers do that". You might want to have a look at ftp://ftp.cs.utexas.edu/pub/garbage/bigsurv.ps which is a fairly recent survey of garbage collection techniques, or at some of the other resources at http://www.cs.ukc.ac.uk/people/staff/rej/gc.html