This article reviews the research literature on sexual fantasy, a central aspect of human sexual behavior. Topics include (a) gender similarities and differences in the incidence, frequency, and content of sexual fantasies and how they relate to sociocultural and sociobiological theories of sexual behavior; (b) the association between frequency or content of sexual fantasies and variables such as age, sexual adjustment and satisfaction, guilt, sexual orientation, personality, and sexual experience; and (c) "deviant" sexual fantasies (i.e., what they are, whether they play a role in the commission of sexual crimes, and whether they can be modified). The article ends with a summary of major findings and suggestions for future research.