Why is bisexuality the object of such scepticism? Why do sexologists steer clear of it in their research? Why has bisexuality, in stark contrast to homosexuality, only recently emerged as a nascent political and cultural identity? Bisexuality has been rendered as mostly irrelevant to the history, politics and theory of sexuality. With this text, Steven Angelides explores the reasons why and invites us to rethink our preconceptions about sexual identity. Retracing the evolution of sexology, and revisiting modern epistemological categories of sexuality in psychoanalysis, gay liberation, social constructionism, queer theory, biology, and human genetics, Angelides argues that bisexuality has historically functioned as the structural other to sexual identity itself, undermining assumptions about homosexuality and heterosexuality.