What part does sexual orientation play in human development? Despite the apparent significance of sexual identities in shaping lives, developmental research and theory on sexual orientation have, until recently, been relatively limited. Today, as lesbian women, gay men, and bisexual people become more open about sexual identities, psychological knowledge about the ways in which developmental processes affect and in turn are affected by sexual orientation is growing rapidly. As the articles in this special issue reveal, groundbreaking research is being conducted, from a variety of perspectives, by investigators from many different traditions. Research on sexual orientation and human development has the potential to make important contributions both to the theory and to the practice of developmental psychology. I wish to thank Deborah Cohn and Anthony D'Augelli for their helpful comments on earlier versions of this article.