Aggression is a term with meanings that range from the expression of mild hostile or angry feelings to behavioral phenomena that result in destructive action. The internal state associated with aggression can be distinguished from its outward manifestations, including violence resulting in physical injury and homicide. After a brief review of what sources are available regarding the prevalence of violence, this article outlines some factors associated with aggression and violence and summarizes current problems in the study of their underlying causes. Individualized assessment is required in order to formulate a treatment plan, whether in the context of clinical care or corrections. Prevention at the individual and societal levels remains an important focus of public health and policy intervention. At present, there is no unified conceptualization of aggressive states and actions leading to the injury of other persons, and research has been feasible only when subtypes of victims and perpetrators have been identified and studied separately. An additional overview of the topic is provided in Aggression: Gender Differences in, which describes female‐perpetrated violence in the context of aggressive behavior in both genders.