For the Greeks and Romans, the world was full of gods, but this fundamental aspect of their experience poses major challenges to modern understanding. The concept of belief has been central to meeting those challenges but has itself been hotly debated, and has at times even been rejected as a supposedly Christianising anachronism. Others, meanwhile, have argued that a culture-neutral model of belief is both possible and essential, while the advent of the cognitive science of religion has offered new possibilities for understanding ancient religious worlds. The essays in this volume trace the historical development of the modern concept of belief, examine ancient debates about the nature of human knowledge of the divine, and draw on perspectives from anthropology, cognitive science and early modern history as well as classical studies to explore the nature and role of belief in Greek and Roman religion in ancient literature, society, experience and practice.