Can participatory democracy only occur at the local level? What happens when participatory democracy goes to scale? The view that modern societies are too big and too complex to be governed by ordinary citizens is ingrained in the social sciences. Even supporters of participatory democracy often assume that it works best in relatively small communities or cities. In the state of Rio Grande do Sul in Brazil, however, politicians and activists have for several years tried to scale up what they call ‘participatory budgeting’ (PB) to the state level. Their experience has been called the world’s largest experiment in participatory democracy. This dissertation draws together evidence from two periods of state PB, and compares this with local level PBs, as well as examining what happened with PB when it came to the Nordic countries. It offers a rare systematic discussion of the challenges of scaling up participatory democracy, as well as an analysis of how PB is translated when it is scaled out. Through four separate articles it makes use of a diverse set of theories – from normative theories of the city and deliberative democracy, to theories of collective action, translation and institutional change – and a combination of methodological approaches including quantitative analysis, original opinion surveys, interviews, comparisons, and text analysis. The dissertation questions the view that participatory democracy only can work at the local level, and makes the argument that scale is not an insurmountable barrier to participatory democracy although there are challenges to ‘scaling up.’ Among these are how to balance representation and grassroots participation, how to include low educated groups in deliberations addressing complex political issues, and how to get politicians and bureaucrats at higher levels of government to delegate power to lay people. There are different ways of confronting these challenges each associated with its own advantages and disadvantages. One novel approach, analyzed here, is the ‘systems turn’ in the practice and theory of participatory democracy. Finally, the dissertation emphasizes that ‘scaling up’ and ‘scaling out’ are intertwined phenomena, and stresses the importance of translations and institutional factors when PB is spread to new localities. At a time of growing disillusionment with the institutions of advanced industrial democracies, there is increasing interest in new ways of involving citizens in the political decisions that affect their lives. An understanding of the challenges involved in scaling up PB should therefore have significance beyond Rio Grande do Sul, and for those who wish to scale up similar participatory endeavors elsewhere.