In many contemporary Occidental societies, bisexuality is paradoxical. Commonly conceived as a combination of heterosexuality and homosexuality, bisexuality as such became conceivable only after the popularization of the hetero/homosexual dichotomy during the late 19th and 20th centuries. Paradoxically, however, the concepts of hetero- and homosexuality, reflecting the cultural belief that an individual's feelings of sexual attraction are naturally directed toward the other sex or, alternatively, toward the same sex, simultaneously renders bisexuality--as an attraction to both genders--inconceivable. In this article, I review the historical and cultural processes that produced the paradoxical conceivability of bisexuality. I then discuss the cultural attitudes toward bisexuality that result from this paradox and show how scientific research on bisexuality has been guided by popular conceptions of, and attitudes toward, bisexuality. Finally, I review efforts to reconceptualize bisexuality for both political and scientific purposes, and summarize recent research on bisexuality using these reconceptualizations. This summary includes research on the prevalence of bisexuality, prejudice against bisexuals, patterns of bisexual behavior, and the meaning of bisexual self-identity.