This chapter introduces the themes and arguments of the book. In particular, it explains the benefits of adopting a holistic approach to the history of the river basin that acknowledges its cultural, physical, and biological unity. Treating the Tigris and Euphrates as a continuous whole brings to light the magnitude and significance of river flow that fostered contacts between upstream and downstream regions. Beyond facilitating communication, the twin rivers formed the backbone of the early modern Ottoman economy in the region by supporting complementary subsistence strategies, such as irrigation agriculture, animal husbandry, and wetland exploitation. In addition to the themes and arguments, this chapter offers a brief introduction to the history of the Ottoman Empire and the ecology of the Tigris-Euphrates basin.