Free will consists in the ability to initiate and execute plans of action. This involves the ability to choose and act in accord with our desires, beliefs, emotions, and intentions by controlling how these mental states issue in our actions. The relevant sort of control consists of conative, cognitive, affective, and physical capacities. This project investigates the neurobiological underpinning of these capacities and the extent to which the brain enables us to have, or prevents us from having, this control. The project consists of two parts: diminishing free will; and enhancing free will. The first part examines how brain dysfunction can compromise the mental capacities necessary for free will, while the second explores interventions in the brain that might restore or enhance these capacities. Neurological and psychiatric disorders can diminish the critical mental capacities by causing dysfunction in the brain mechanisms that mediate them. The mental impairments caused by brain dysfunction usually come in degrees. Free will is not always a capacity that a person either has or lacks but often falls along a spectrum of control. What matters with respect to where one falls on this spectrum is not brain dysfunction as such but how it can compromise one's ability to reason and act. When these capacities are impaired, pharmacological, psychological, and surgical interventions may restore them by modulating the underlying neural pathways. Pharmacological interventions may also enhance normal levels of these capacities. I will undertake this project as an interdisciplinary endeavor, drawing from neuroscience, philosophy, psychology, and law and discussing the key issues with experts in these fields. My lectures, single- and co-authored papers, and an edited book will contribute to a deeper understanding of free will and its implications for philosophy, mental health, and the criminal law.