People who are not content to remain the gender they were assigned at birth have existed throughout human history and in all recorded cultures (Herdt, 1996). Naturally the experiences of people in the contemporary high-income cultures which are the focus of this book will be informed by the understandings and technology available now, but fundamentally gender diversity is not a new phenomenon. It is therefore curious that, until recently, it was commonly thought that gender diversity was so rare that it would not trouble most psychiatrists or psychologists in their usual course of work. This is no longer the case, as there has recently been an exponential increase in the number of gender diverse people coming forward, which mirrors that of same-sex-attracted people a couple of decades before. Most likely this is once again due to the greater, although still tenuous, social acceptance being shown towards sexual and gender diversity in some countries. It follows therefore that psychiatrists and others will have seen gender diverse people throughout their careers, but without necessarily being aware that they were doing so.