Focusing primarily on academic feminist theory, this chapter reviews the historic importance and challenge of defining a “woman” for the purposes of feminist theorizing and politics. Part of that history is about whether and how to include transgender women. So-called TERFs are “trans-exclusionary radical feminists,” or “gender critical feminists.” My intervention in this chapter is not designed to resolve all the debates (a practical impossibility), but rather to note why such debates are likely to continue, and to suggest that the concept of regulatory definitions might enhance the conversation.