This paper analyzes police reform and its relationship to the development of democracy. To this end, it develops a model that explains police reform by three independent variables: (1) stage of democracy, (2) dimension of change, and (3) type of intervention. The methodology employs the review of the literature about police reform in the United States, Britain and Latin America to test the model. It also examines police reform in Brazil to study the demilitarization of the police force.