We review the effects of androgen deprivation on the sexual behavior of human males. Although eunuchs have existed in many cultures over the last 4,000 years, there is scant detailed and specific information in the historical record about castration status and sexual behavior. From the literature on modern-day eunuchs who are not sex offenders, we conclude that androgen deprivation reduces sexual desire and behavior, including sexual intercourse. Most men, especially those who did not volunteer for the treatment, experience the side effects as extremely bothersome. Androgen deprivation therapy (ADT) receives endorsements from some clinicians who treat sex offenders, and it probably reduces sexual recidivism among men who freely request the procedure, but good evidence is sorely lacking. Men who freely request and persist with ADT are probably an especially low-risk group. Little is known about the effects of sexual or violent recidivism among sex offenders who do not freely request it. Little is known about the long term effects of ADT on sexual behavior in general, and sexual recidivism in particular, or about long-term health effects. Clearly, much more research is needed before ADT has a sufficient scientific basis to be relied upon as a principal component of sex offender treatment. (PsycINFO Database Record (c) 2011 APA, all rights reserved)