This paper examines the energy partnership between Turkey and Azerbaijan in the context of the Southern Corridor. It argues that an interdependent relationship between Turkey and Azerbaijan has emerged, parallel to the rising importance of the Caspian basin in European energy security discussions. It maintains that Azerbaijan plays three distinct yet interrelated roles in Turkey’s energy policies: supplier for domestic markets, contributor/enabler of transit projects, and investor. The paper investigates the development of Turkey-Azerbaijan relations chronologically. It shows how these three roles evolved in three successive phases: the 1990s; the first decade of the 2000s; and recent years. The paper then analyzes the implications of the recent TANAP+TAP project for Turkey-Azerbaijan relations as well as for the Southern Corridor in general.