Recent upheaval in the global energy system -- dramatic increases in demand led largely by developing countries, significant decreases in supply as a result of local or regional conflicts, and the growing nexus between the burning of hydrocarbons and climate change -- has unsettled long-held notions of energy security. For many years, transatlantic cooperation helped undergird the system's stability, but Europe and North America have drifted apart in several key ways, potentially undermining the search for energy sufficiency, surety, and sustainability. Will the transatlantic partners continue on separate paths in the face of dramatic change in the global energy system, or does the breadth and depth of the challenges they confront compel them to work more closely together? In this edited volume, experts from across Europe and North America -- including advisors to the executive and legislative branches of both the EU and the United States, to senior military commanders, and to major international organizations and companies -- examine the most salient facets of the transatlantic energy relationship and discern whether that relationship is characterized by growing convergence or divergence.