The chapter presents the theoretical argument of the book. First, it proposes a typology of anti-austerity political projects that achieved a dominant – or, at least, electorally relevant – position within the Lefts in the aftermath of major socioeconomic crises in selected countries in Latin America and Southern Europe. The typology is built on three branching variables, namely: the previous existence, or not, of the political party and/or of partisan structures, their relationship with the national unions and the degree of influence social movements have in crucial internal activities of the parties. Four ‘outcomes’ are identified: the resilience of ‘labour-based Left’ or the emergence of three kinds of anti-neoliberal populisms – movement-based populism, party-rooted populism and leader-initiated populism. Two ‘sets’ of causal forces, according to the argument, produce divergent outcomes and are extensively discussed in the chapter. One of these sets (the ‘critical antecedents’) operates mainly before the crisis; the other (consisting in the characteristics of anti-austerity protest cycles in each country) operates during (and in reaction to) the crisis and the austerity measures. The critical antecedents help us to understand the subsequent evolution of the national political scenarios as they affect the adaptability of pre-existing left-wing parties to the post-crisis scenario. The characteristics of the main social mobilisations against the neoliberal model, jointly with the critical antecedents, create the political opportunities for the (eventual) emerging and consolidation of different varieties of anti-neoliberal populist parties. A methodological section concludes the chapter.