This study aims at exploring the significance of maritime commerce to societies on the Indian Ocean rim, by examining how rulers adjusted their policy in order to control and profit from trade. The point of departure is the anonymous Greek first century AD Periplus of the Erythraean Sea. This is a guide to navigation and trade on the Indian Ocean, covering the coasts of the Red Sea, Gulf of Aden, East Africa and India. The unknown author, who to a large extent relied on personal experience, included not only sailing directions, but also a wealth of information on local products, markets and political conditions.