Social movements face important challenges when sections of them evolve into political parties. These dilemmas may become quite extreme when the first electoral contest brings them into the local government of the most important cities, as has happened in Spain. We will use this exceptional case to reflect on how social movements address this two-step process (from movements to party and from party to government). First, we will discuss organisation issues, including the formalisation of permanent organisational structures or leader selection. Second, participation dilemmas: how a movement that had democracy as its central political demand manages participatory policies once it governs large cities. To address these issues, we will focus mostly on the cases of Madrid and Barcelona (2015–2018). The two cases show similarities but also major differences: contextual factors and the different composition of the socio-political coalitions resulted in distinct organisational and democratic strategies and practices. The chapter will be based on different types of evidence, from secondary sources to reports and interviews from a research project on participatory institutions in these cities.