The economic development of Greece and Rome hinged directly on the ability of commercial vessels to transport large volumes of goods across the Mediterranean and Black Sea. Archaeology has revealed the sizes, construction methods and cargos of these ships, but the navigational techniques that were employed to direct them from port to port remain unclear and elusive. In ancient literature, the oft-repeated themes of storm, shipwreck and death at sea led to the popular assumption among scholars that seafarers developed habits to minimize their exposure to this hostile element--hugging the shore to avoid the open sea, putting in at night, sailing only in summer, and using 'seafaring manuals' to help guide their way. While several recent studies have made some strides in overturning this overly simplistic view by highlighting aspects of navigation in certain areas and in certain periods, the 'standard model' lingers in both scholarly and popular imagination. This study offers a comprehensive review of the scattered textual and archaeological evidence pertaining to ancient seafaring and navigation, and a major reinterpretation of ancient commercial navigation in both periods. Chapters 2-3 explore the parameters of the maritime environment (coasts, winds, currents and visibility) and the human responses to them in the form of ships, seasonal rhythms and maritime corridors. Chapters 4 and 5 discuss the ways in which Greek and Roman sailing masters accounted for the fundamental requirements of navigation--the determination of direction, position, speed and distance--using wind roses as a 'compass' and various stars and star groups at night. Chapter 6 treats the question of whether seafarers used written guides or experience, or both, to help determine their position. Chapter 7 explores the historical figure of the sailing master himself and integrates a wide range of evidence to reconstruct the navigational routines of the crews of Alexandrian grain ships during the Roman imperial era. My research concludes that both coastal and open-sea sailing were matters of routine in the commercial sector, that commercial seafarers did indeed sail at night and employ the stars to deduce navigational information, that winter sailing was a widespread practice, and that crews employed navigational strategies to weather storms, usually successfully. / text