The ascendance of austerity policies and the protests they have generated have had a deep impact on the shape of contemporary politics. The stunning electoral successes of SYRIZA in Greece, Podemos in Spain and the Movimento 5 Stelle (M5S) in Italy, alongside the quest for a more radical left in countries such as the UK and the US, bear witness to a new wave of parties that draws inspiration and strength from social movements. The rise of movement parties challenges simplistic expectations of a growing separation between institutional and contentious politics and the decline of the left. Their return demands attention as a way of understanding both contemporary socio-political dynamics and the fundamentals of political parties and representation. Bridging social movement and party politics studies, within a broad concern with democratic theories, this volume presents new empirical evidence and conceptual insight into these topical socio-political phenomena, within a cross-national comparative perspective.