The study of democracy -- how it emerges, flourishes, and fails -- is as old as Aristotle. In the last half century, as democracy has spread around the world, empirical and theoretical inquiry into its sources and prospects has rapidly expanded into a vibrant scholarly field. This volume is an engaging survey of what is known and not known about the causes and consequences of democratization. Theories about this process have evolved, from an early emphasis on the long-term impersonal forces of modernization and economic development to a later focus on leadership, civil society, and the breakdown of authoritarianism. The “third wave” of democratization, which began in the late 1970s, made scholars look more closely at the international diffusion of democratic movements and impulses. More recently, the troubles encountered by some new democracies have drawn attention to the social and economic sources of crisis. Three main lessons have emerged: there is no single path to democratization, the relationship between economic development and democracy is complicated, and a supportive international environment can help, but not ensure, democratic breakthroughs.