The brief first notes the difficulty in estimating the number of sex crimes committed because of low levels of reporting; however, statistics on the incidence and prevalence of sex crimes, as well as trend data, provide insight into the nature and extent of sexual violence, which assists policymakers and practitioners in designing and delivering more effective prevention and intervention strategies. Data from law enforcement and victimization surveys suggest that sexual assaults have substantially declined over the past 10 to 20 years, as have other types of crime. According to the FBI, the number of forcible rapes reported to the police decreased 14 percent between 1990 and 2009, from 102,555 to 88,097. The number of rapes reported to police per 100,000 U.S. residents also decreased 30 percent during this period. Data on sexual assault victimization surveys show a similar pattern. This brief advises that policymakers, while recognizing data limitations, should monitor key indicators of the incidence of sexual offenses over time and work with researchers to better understand the factors that influence data patterns, including the roles of various policies and practices that are designed to prevent, treat, or otherwise intervene in sexual offending. Hidden sexual offending will continue to pose a significant problem in data collection and analysis; therefore, practitioners must recognize and acknowledge these limitations in measuring outcomes of sex offender management. This brief describes the methodologies, strengths, and weaknesses of various data-collection instruments being used to measure sexual offenses. 16 references