This book provides a study of regret (metameleia) in the moral psychology of Plato, Aristotle, and the Stoics. It was important for all these philosophers to insist that regret is a characteristic of neither fully virtuous nor wholly irredeemable characters. Rather, they took regret to be something that affects people who retrospectively feel pain at realising an earlier mistaken action. Regret sets out in full the accounts of the nature of this emotion found in the works of these philosophers, viewing them in the context of their respective accounts of virtuous and non-virtuous agents, ethical progress, the role of knowledge in producing good actions, and compares it with modern philosophical notions of 'agent regret'.