This thesis constitutes one of the first attempts to explore the concept of partisan affective polarization - that has hitherto been studied almost exclusively in the United States - in comparative cross-national perspective. I outline a conceptualization of affective polarization in multiparty context as the mean difference between voters’ in- and out-party feelings across all relevant party dyads. Based on this definition, I propose a novel measure – the Affective Polarization Index (API) – to estimate the degrees of affective polarization. The API scores reveal that affective polarization is acutely present in most countries across the world, and that in many cases it is even higher than in the United States. Subsequently, I study the foundations of this phenomenon, building on the rational and social identity (’tribalism’) theories. The system level analysis shows that the two most prominent variables in current research (ideological divergence and partisan identity strength) are insufficient to unravel why countries exhibit differing levels of affective polarization, although the ideological and affective manifestations of polarization are significantly correlated. A substantial part of the remaining cross-national variation is explained by the quality of governance and the ethnic composition of the societies, as countries with corrupt ineffective governments and high levels of ethnic heterogeneity are more polarized on affective terms. Finally, I examine the individual level predictors of affective polarization. The findings from the case study of Sweden demonstrate that affective polarization aligns with the ideologically tripolar configuration of the party system: while partisan feelings between the mainstream right and left blocs are mostly driven by socioeconomic positions, the affect between the right-populist and mainstream parties correlates more with cultural issue stances. However, hostility towards the right-populist party is stronger than the extent of ideological disagreement predicts, and appears to be exacerbated by the high levels of trust towards the central democratic institutions among the supporters of the mainstream parties. Overall, this dissertation suggests that affective polarization has systematic underpinnings that relate to both rational and tribalist motivations.