This article considers the notion of free will in the context of aggression and psychopathy research. The philosophical literature is very briefly considered to determine under what assumptions free will can be considered to exist. However, as the issue of free will is very difficult to address directly, the prime focus of this article is on issues raised in the philosophical debate, that may be empirically tractable and that are relevant to the understanding of psychopathy. Specifically, the following issues are considered: (1) The distinction between automatic and controlled processing; (2) Impairment related to automatic processing in individuals with psychopathy; and (3) Impairment related to controlled behavior in individuals with psychopathy. It is concluded that, while there is not a direct mapping of the automatic versus controlled processing dichotomy on to the reactive versus instrumental aggression dichotomy, some overlap can be considered. As such, it is possible to consider that certain episodes of reactive aggression might be considered to occur in the absence of free will. However, instrumental aggression, at least from a compatibilist perspective, must involve free will. Published in 2007 by John Wiley & Sons, Ltd.