The aim of this volume is to assess the cartel party theory based on a study of the Swedish party system, specifically, the party groups in the unicameral Swedish Parliament (the Riksdag). Attending to a range of factors, we ask whether or not the party groups in the Riksdag have increasingly come to resemble one another in the period from the late twentieth century to the first decade of the twenty-first century. In this chapter of introduction, we present the Swedish case as well as six empirical studies of the Swedish case highlight various factors that the cartel party theory takes to be conducive to cartelisation, and discuss countervailing forces as well. We explore whether party elites are becoming increasingly alike and increasingly removed from grassroots and constituents due to processes of professionalization; whether opposition to government is waning in the wake of globalisation and Europeanisation; and whether parties are becoming more alike by adapting to general processes of mediatisation in politics. In two chapters, we investigate key factors not yet sufficiently acknowledged either by authors or critics of the cartel party theory: gender representation and party culture; we ask whether these factors act as countervailing forces to cartelisation. In a concluding chapter we discuss how the empirical results presented here might lead us to rethink accepted ideas about cartelisation, convergence and increasing similarities among parliamentary parties.