In recent years, the idea that each person is in a privileged position to acquire knowledge about her own mental states has come under attack. A growing body of empirical research has cast doubt upon the existence of what philosophers call 'first person self-knowledge', i.e., knowledge about our mental states that is often thought to be immediate, transparent, and authoritative. This line of thoughthas led some philosophers to claim that what seems to be 'first-person self-knowledge' is really just 'third-person self-knowledge,' i.e., knowledge about our mental states that is inferential, opaque, and fallible. This book discusses challenges for first-person knowledge and explores the true nature of third-person knowledge.