This thesis aims to explain levels and changes in levels of political participation through institutions and institutional change. Three general trends have been distinguished; a decline in ‘representative’ participation, an increase in ‘extra-representative’ participation, and an increase in ‘direct’ participation. The notion that these trends are apparent in most established democracies is rather striking and puzzling. This study specifically contributes to the debate about the current state and health of democracy, considering that it is concerned with both system changes in democracies and levels of popular participation in these democracies. Changes in these types of participation could partly be explained by changes in the structure of the political system. Although the notion that institutions matter in political life is not debated, and their effects have been examined before, the effects of institutions have rarely been examined structurally in large-scale analyses, specifically concerning participation. In this study, three institutional elements which affect the access points and inclusiveness of the political system are considered: ‘horizontal’ diffusion of responsibilities (privatisation; the creation of non-majoritarian institutions), ‘vertical’ diffusion of responsibilities (decentralisation; globalisation) and ‘diagonal’ diffusion of responsibilities (direct democracy institutionalisation). The main aim of this research is therefore to find to what extent changes in the political system influence levels of different forms of political participation, over time and across space. The study finds that, generally, representative participation seems to be negatively affected by privatisation, non-majoritarian institutions, globalisation, and the institutionalisation of referendums, with unclear results regarding the impact of decentralisation. Extrarepresentative participation is generally positively influenced by privatisation, nonmajoritarian institutions, decentralisation, and globalisation, while there are some mixed results concerning the influence of the institutionalisation of referendums. Finally, direct participation is negatively affected by privatisation, non-majoritarian institutions, decentralisation, and globalisation, while referendum institutionalisation has generally a positive effect on levels of direct participation.