Susanne Bobzien presents the definitive study of one of the most important intellectual legacies of the ancient Greek world: the Stoic theory of causal determinism. She identifies the main problems that the Stoics addressed and reconstructs the theory. She discusses how the Stoics squared their determinism with their conceptions of possibility, action, freedom, and moral responsibility, and how they defended it against objections and criticism by other philosophers. She shows how the Stoics distinguished their causal determinism from ancient theories of logical determinism, fatalism, and necessitarianism. Along the way she gives an authoritative account of many other related aspects of Stoic thought, including their views on the predictability of the future, the role of empirical sciences, the determination of character, and moral freedom. Bobzien's study of these central doctrines of Stoicism reveals the considerable philosophical richness and power that they retain today.