We review the literature on the relation between socio-economic development and political democracy, a field that is commonly known as modernization theory. Guided by the seminal contribution of Lipset (1959), we assess the evolution of this literature along two major dimensions: (1) robustness of the relationship between economic development and democracy and (2) substantiation of the causal mechanism. The evidence to date suggests that Lipset’s original thesis does indeed find empirical support, and that certain structural conditions are conducive to stable democracy.