For at least a thousand years Greek cities took part in religious activities outside their territory by sending sacred delegates to represent them. The delegates are usually called theōroi, literally 'observers', and a delegation made up of theōroi, or the action of taking part in one, is called theōriā. This is the first comprehensive study of theōroi and theōriā. It examines a number of key functions of theōroi and explains who served in this role and what their activities are likely to have been, both on the journey and at the sanctuary. Other chapters discuss the diplomatic functions of theōroi, and what their activities tell us about the origins of the notion of Greek identity and about religious networks. Chapters are also devoted to the reception of the notion of theōriā in Greek philosophy and literature. The book will be essential for all scholars and advanced students of ancient religion.