  This entry is about two kinds of circularity: object circularity, where an object is taken to be part of itself in some sense; and definition circularity, where a collection is defined in terms of itself.  Instances of these two kinds of circularity are sometimes problematic, and sometimes not.  We are primarily interested in object circularity in this entry, especially instances which look problematic when one tries to model them in set theory.  But we shall also discuss circular definitions.