  This entry has two major thrusts.  First, it introduced non-wellfounded sets and described some of the mathematics around them.  It was not comprehensive in this regard, and one should see Peter Aczel’s book (1988) for much more, including discussions of axioms besides AFA that we did not even mention. One could also see Barwise and Moss (1996) for more on some of the points that were touched on  here.[13]  The presentation owes much to work in coalgebra that began shortly after these books appeared.  So readers familiar with either of them would still find something new in the first sections of the entry.