The claim that war is in decline has gained considerable traction among analysts and policy makers. This review surveys the empirical data as well as the theoretical arguments for a decline of war. It concludes that, while great power wars have declined since 1945, war in general is not necessarily in decline. Critiques of the claim that war is in decline range from the statistical to the anthropological. While war is not necessarily in decline, though, it has changed. Specifically, a major driver of the putative decline of war—international norms, laws, and institutions—has done more to change how war is discussed and presented than whether it is prosecuted. Stronger theories of change are needed in international relations to understand whether these somewhat superficial changes could deepen in ways that produce a true decline in war.