As thinkers in the market for knowledge and as agents aspiring to free, morally responsible action, we are inevitably subject to a wide range of different kinds of luck. Once appreciated, luck's pervasive influence on human intellectual and practical endeavor can become a source of acute distress. Our inevitable subjection to luck threatens to thoroughly frustrate our aspirations to knowledge and free, responsible agency, thereby sapping much of the value from our intellectual and practical successes. This book presents and defends a comprehensive new theory of luck in light of a critical appraisal of the literature's leading accounts, then brings this new theory of luck to bear on central issues in the theory of knowledge and the philosophy of action. It argues for an optimistic view of luck's significance for human intellectual and practical endeavor according to which knowledge and free, responsible agency are compatible with a surprisingly wide range of luck-related phenomena.