Free will has often been considered central to criminal responsibility. Yet, the concept of free will is also difficult to define and operationalize, and, moreover, it is intensely debated. In particular, the very existence of free will has been denied based on recent neuroscience findings. This debate has significant implications on those fields in which the link between free will and behaviour is the main focus of interest, such as forensic psychiatry. In fact, a tension is often experienced between the centrality of the notion of free will on the one hand, and its controversial status on the other. This tension needs to be addressed, especially in forensic psychiatry, since it is relevant for actual assessments of legal insanity. In the present paper we will try to operationalize “free will” using a fourpartite decision-making capacity model, which can be used in forensic assessment of insanity. We will describe its advantages and application to guide mental insanity assessments. Whereas free will is often considered problematic from a neuroscience perspective, this model, we argue, is compatible with neuroscience; moreover, evaluations using this model can also be informed and strengthened by neuroscientific findings, for example regarding inhibitory control.