This study deals with the quality of democracy, and its purpose is to examine which factors affect the varying levels of the quality of democracy in the stable democracies in the world. The research question posited in the study is: what explains the varying levels of the quality of democracy in the democratic countries in the world, and do political institutions matter? Theoretically, the quality of democracy is distinguished from other similar concepts employed in comparative politics, and what the quality of democracy stands for is clarified. The quality of democracy is defined in this study as: the level of legitimacy in a democratic system with respect to democratic norms such as political participation, political competition, political equality, and rule of law. In total, four dimensions of the quality of democracy are included that are considered to be very important dimensions of the quality of democracy. These dimensions are political participation, political competition, political equality, and the rule of law. To explain the variation in the quality of democracy, an explanatory model has been developed. The explanatory model consists of five different groups of independent variables: political institutional variables, socioeconomic variables, cultural variables, historical variables, and physical variables. Methodologically, a large-n, outcome-centric research design is employed and statistical analysis is used to examine what effect the five groups of independent variables have on the four dimensions of the quality of democracy. Empirically, the results show that cultural variables and political institutional variables outperform socioeconomic, historical, and physical variables in relation to their effect on the quality of democracy. Consequently, cultural and political institutional variables are the two most important groups of variables when explaining the variation in the quality of democracy in the democratic countries in the world. In relation to the other groups of variables, historical variables are slightly more important than socioeconomic variables when explaining the variation in the quality of democracy. The physical variables constitute the group of variables that has the least importance out of the five groups of variables when explaining the variation in the quality of democracy. In summary, the findings from the study show that the best way of increasing the level of the quality of democracy may be to choose political institutions such as parliamentarism as the executive power system and a proportional system as the electoral system. To put this clearly, to increase the possibility of democratic countries achieving a high level of the quality of democracy they should avoid majority electoral systems and presidential or semipresidential executive systems.