Sexual classification systems are based on precise and understandable definitions of sexual dysfunctions and are needed for investigative research, determination of diagnostic standards, and delineation of treatment strategies. The four major categories of sexual dysfunctions include disorders of sexual desire/interest, arousal, orgasm, and sexual pain. The purpose of this article is to review the major features, differences, and similarities of the six classification systems widely used in sexual medicine, including the International Classification of Diseases, the Diagnostic and Statistical Manual of Mental Disorders, the National Institute of Health Consensus Conference on Impotence, the American Foundation for Urologic Diseases, International Consensus Conference on Women’s Sexual Dysfunction, and the First and Second International Consultations on Sexual Dysfunctions. Hatzimouratidis K, and Hatzichristou D. Sexual dysfunctions: Classifications and definitions. J Sex Med 2007;4:241–250.