This comprehensive study offers a balanced assessment of libertarian accounts of free will. Bringing to bear recent work on action, causation, and causal explanation, Clarke defends a type of event-causal view from popular objections concerning rationality and diminished control. He subtly explores the extent to which event-causal accounts can secure the things for the sake of which we value free will, judging their success here to be limited. Clarke then sets out a highly original agent-causal account, one that integrates agent causation and nondeterministic event causation. He defends this view from a number of objections but argues that we should find the substance causation required by any agent-causal account to be impossible. Clarke concludes that if a broad thesis of incompatibilism is correct--one on which both free will and moral responsibility are incompatible with determinism--then no libertarian account is entirely adequate.