A defendant is criminally responsible for his action only if he is shown to have engaged in a guilty act—actus reus (eg for larceny, voluntarily taking someone else’s property without permission)—while possessing a guilty mind—mens rea (eg knowing that he had taken someone else’s property without permission, intending not to return it)—and lacking affirmative defenses (eg the insanity defense or self-defense). We therefore first review neuroscientific studies that bear on the nature of voluntary action, and so could, potentially, tell us something of importance about the actus reus of crimes. Then we look at studies of intention, perception of risk, and other mental states that matter to the mens rea of crimes. And, last, we discuss studies of self-control, which might be relevant to some formulations of the insanity defense. As we show, to date, very little is known about the brain that is of significance for understanding criminal responsibility. But there is no reason to think that neuroscience cannot provide evidence that will challenge our understanding of criminal responsibility.