An understanding of the epidemiology and phenomenology of nonsuicidal self-injury (NSSI) is vital for researchers, clinicians, and policymakers interested in this problem. The outcomes of recent studies have provided data that answer basic questions concerning these aspects of NSSI. For example, rates are much higher among adolescents and young adults than among children and older adults. Also, studies demonstrate higher rates among females, although the behavior is more common than previously thought among males. In this chapter, we provide an overview of the information currently available concerning the epidemiology and the phenomenology of NSSI. We conclude with an agenda for future work in this area directed at addressing gaps in out current knowledge.