This chapter discusses the theory of determinism put forward by the ancient Stoics and its theory for rational action and moral responsibility. The Stoic argument for determinism is presented in Section 1. Stoic determinism implies fatalism. The first problem, studied in Section 2, is whether it is rational to be motivated to do anything if one believes in fatalism. A second problem is that determinism seems to imply that everything people do is fully determined by external causes alone. This problem, (...) and the solution proposed by the Stoics, are studied in Section 3. A third problem, discussed in Section 4, is whether moral responsibility is compatible or incompatible with the possibility of alternate actions. The chapter discusses two objections raised by incompatibilists, which relate to the notion of agency and freedom from external force. The chapter briefly considers one possible counter‐objection to Chrysippus, the third head of the Stoic school. (shrink)