Summary of doctoral thesis: My doctoral thesis investigates the moral psychology of laughter in ancient philosophy. Although the first book by a notable philosopher on laughter was written by Henri Bergson in 1900, laughter demanded serious philosophical attention from nearly all of the most influential Greek philosophers: comments on laughter in the works of Plato, Aristotle, Epictetus, and Epicurus are embedded in wider discussions of moral psychology, ethics, biology, politics and literary criticism. By investigating these philosophers’ explicit “theories” of laughter, as well as their employment of laughter/humour in their own writings, my aim is to highlight the double-sided potential of laughter i.e. its potential for both undermining and upholding the authority of reason in the agent’s soul. And by exploring how Plato and Aristotle both foreshadow and diverge from the modern Superiority, Incongruity, Relief and Play theories of laughter/humour, I hope to increase our contemporary understanding of why we laugh.