International alliances are the most important security institutions in international relations. This chapter examines different types of alliances, including security pacts, nonaggression agreements, and consultation pacts. It then connects alliances to more established ideas about balance of power dynamics. The next part of the discussion centers on the question, why do we observe states complying with alliance agreements, under conditions of international anarchy? Some possible answers are: some international alliance treaties increase coalition fighting power; states and leaders risk paying international and domestic audience costs if they violate the terms of an alliance agreement; and states only sign alliance treaties they mean to comply with, which means they are deliberate in how they design alliance treaties. Other concepts and debates under discussion include balancing, hands-tying, bandwagoning, free-riding, military bases, and power projection. Many of these concepts are applied to a quantitative study on whether the presence of an alliance agreement makes a state more likely to intervene on behalf of another state, and a case study of World War I.