Alliances are subject to many scholarly inquiries in international relations and peace research because they are major instruments of foreign and security policies. Since the early work on alliances produced by the Correlates of War (COW) project, there have been significant advances in conceptual, theoretical, and empirical alliance research. New typologies and data permit us to differentiate more thoroughly among a variety of alliance objectives and functions. Furthermore, there has been new theoretical and empirical research on alliance formation, alliance configuration/polarization, effects of alliances on military conflict, connections between alliances and trade, and the economics of alliances. Providing new theoretical approaches, data, and empirical evidence on alliances, this special issue includes articles that address alliance formation, alliance polarization, alliances and democratization, trade among allies, regional economic institutions with alliance obligations, and defense industrial policies of military alliances. The articles in this issue extend our understanding of alliances past the traditional realist balance-of-power framework and encourage further testing and refinement of older alliance arguments and extensions to new theoretical developments.