The increasing recognition of the large number of people victimized by sexual assault has focused considerable attention on methods for deterring its perpetrators. In particular, law enforcement and mental health professions alike seek ways to prevent sex offenders from repeating their criminal behavior. Some evidence suggests sex offenders often continue offending even after incarceration or clinical treatment. However, recidivism rates for sex offenders are unusually hard to establish, owing to gross underreporting of sex crimes. This article presents a comprehensive review of empirical studies of sex offender recidivism. It brings together data from a wide variety of studies on both treated and untreated sex offenders of all types. Because of the variety and gravity of methodological problems in these studies, guidelines are discussed for sample selection and description, study design, criterion assessment, and data analysis. With these methodological principles in mind, the results of some 42 studies are examined for what they can tell us about sex offender recidivism and efforts to reduce it.