This book analyses the practice and the theorisation of populism by radical left parties based on a comparative analysis between two cases: Podemos (We Can) in Spain, and La France insoumise (Unbowed France, LFI) in France. This comparative analysis, first of its kind, aims to transform our understanding of left-wing populism at a time when parties of the populist left have succeeded in, or are on the verge of, taking power. The project is timely as, in Europe, the rise and electoral breakthroughs of the radical left in the years 2010-2020 have been based on populist elements - that is, on a rhetoric (1) opposing the people to the elite and (2) demanding a 'radicalisation' of democracy through the concept of popular sovereignty. For each party, the book focuses on the perspective of both political elites (i.e. leaders) and activists examining their motivations and their attitudes towards the 'populist' label. This book will be of interest to researchers of populism, the radical left, and political parties.