This article introduces our themed section on The Left(s) and Nationalism(s), which provides a comparative analysis of the relationship between nationalism and different left-wing parties in Western Europe. It highlights the innovative comparative perspectives offered by this themed section, which not only concerns a series of different geographical cases studies but also involves the ideological plurality of the Left. The larger research question that our contributors address is how different left-wing parties have dealt with the inherent ideological tension between the universality claimed by the Left and the particularism inherent in nationalism, as a doctrine and a principle of political legitimacy. The article stresses three main contributions of our themed section: (1) Western European left-wing parties do engage with the themes of nationalism and nationhood, but they often rely on convenient silence to solve some of the contradictions with their progressive ideology. (2) None of these parties have formulated thick versions of the respective national identities. (3) State-wide left-wing parties have used instrumental conceptions of nationhood to address the challenge of separatist parties, but only with mixed results.