Although it is widely believed that risk assessment tools can help manage risk of violence and offending, it is unclear what evidence exists to support this view. As such, we conducted a systematic review and narrative synthesis. To identify studies, we searched 13 databases, reviewed reference lists, and contacted experts. Through this review, we identified 73 published and unpublished studies (N = 31,551 psychiatric patients and offenders, N = 10,002 professionals) that examined either professionals’ risk management efforts following the use of a tool, or rates of violence or offending following the implementation of a tool. These studies included a variety of populations (e.g., adults, adolescents), tools, and study designs. The primary findings were as follows: (a) despite some promising findings, professionals do not consistently adhere to tools or apply them to guide their risk management efforts; (b) following the use of a tool, match to the risk principle is moderate and match to the needs principle is limited, as many needs remained unaddressed; (c) there is insufficient evidence to conclude that tools directly reduce violence or reoffending, as findings are mixed; and (d) tools appear to have a more beneficial impact on risk management when agencies use careful implementation procedures and provide staff with training and guidelines related to risk management. In sum, although risk assessment tools may be an important starting point, they do not guarantee effective treatment or risk management. However, certain strategies may bolster their utility.