Natural law theory has been undergoing a revival, especially in political philosophy and jurisprudence. Yet, most fundamentally, natural law theory is not a political theory, but a moral theory, or more accurately a theory of practical rationality. According to the natural law account of practical rationality, the basic reasons for actions are basic goods that are grounded in the nature of human beings. Practical rationality aims to identify and characterize reasons for action and to explain how choice between actions worth performing can be appropriately governed by rational standards. These standards are justified by reference to features of the human goods that are the fundamental reasons for action. This book is a defence of a contemporary natural law theory of practical rationality, demonstrating its inherent plausibility and engaging systematically with rival egoist, consequentialist, Kantian and virtue accounts.