As simple as the title of the present chapter may sound, as difficult it is to sum up the many different theories and findings related to sexual behavior. One of the reasons may be the fragmentation of research on sexual behavior across many different disciplines within and outside of psychology: There is, for instance, clinical research on maladaptive or deviant sexual behavior, there is social psychological work on close relationships and gender roles, and there is psychophysiological research on aggressiveness and permissiveness in sexual behavior. Apparently, it seems difficult to find a common denominator of these different approaches.