The paper initially focuses on explanations of police-citizen violence drawn from the author's experiences in testifying at a trial to decide responsibility for a long-standing pattern of police actions ranging from brusqueness to beatings and shootings in which a subset of police officers were disproportionately represented. This discussion highlights research findings that pose problems for the situation theory of violence and its emphasis on victim-related variables. The author then suggests that many catalytic situations to which violence responds can be viewed as interpersonal encounters that evolve cumulatively, but reflect pre-existing psychological traits or states of the victims and/or aggressors. A threefold typology of violence-related situations is outlined: violence opportunities, where violence-prone traits pre-exist in one person so that almost any response in the victim suffices to bring violence about; violence transactions, encounters in which one participant's behavior is deemed sufficiently offensive by the other to inspire a nonviolent but hostile response which can escalate into violence; and violence situations, in which violence is required for survival and personal traits are deemed of no interest. Implications of this model for violence causation theories are explored. Charts, footnotes, and approximately 60 references are supplied.