Free will consists in the capacity to initiate and execute plans of action. It involves the capacity to respond to reasons and control how our motivational states issue in our actions. Neurological and psychiatric disorders can diminish this capacity by causing dysfunction in the neural networks that mediate it. But brain abnormalities do not necessarily compromise free will. Pharmacological, surgical, and psychological interventions can restore the relevant capacity to varying degrees. The most difficult questions regarding free will involve conscious and unconscious aspects of our mental states and their role in self-control. I discuss violent impulses, addiction, and psychopathy and consider whether or to what extent individuals with these conditions can control their behavior and be responsible for it. Certain drugs might enhance the will and responsibility by increasing choices and strengthening responsiveness to reasons. While we should be open to this idea, it can have counterintuitive consequences.