Abstract. Sexual scientists have recognized for over a century that biologic males who seek sex reassignment – male-to-female (MtF) transsexuals – are not a homogeneous clinical population but comprise two or more distinct subtypes with different symptoms and developmental trajectories. The most widely used typologies of MtF transsexualism have been based on sexual orientation and have distinguished between persons who are androphilic (exclusively sexually attracted to males) and those who are nonandrophilic (sexually attracted to females, both males and females, or neither gender). In 1989, psychologist Ray Blanchard proposed that most nonandrophilic MtF transsexuals display a paraphilic sexual orientation called autogynephilia, defined as the propensity to be sexually aroused by the thought or image of oneself as a woman. Studies conducted by Blanchard and colleagues provided empirical support for this proposal, leading to the hypothesis that almost all nonandrophilic MtF transsexuals are autogynephilic, whereas almost all androphilic MtF transsexuals are not. Blanchard’s ideas received increased attention in 2003 after they were discussed in a book by psychologist J. Michael Bailey. The concept of autogynephilia subsequently became intensely controversial among researchers, clinicians, and MtF transsexuals themselves, causing widespread repercussions. This article reviews the theory of autogynephilia, the evidence supporting it, the objections raised by its critics, and the implications of the resulting controversy for research and clinical care.