This dissertation focuses on legislative party switching which occurs when members of Parliament (MPs) change party affiliation during the course of a parliamentary term. Switching is both normatively relevant (as it has the potential to undermine the democratic process), and theoretically significant (as it is linked to the general question of why parties exist and change). The aim of the thesis is three-fold. First, it assesses the importance and scope of the phenomenon in question. By collecting a unique data set of all the inter-party defections that occurred in a sample of 14Western European countries from 1945 to 2015, the thesis describes how party switching has evolved over time and the patterns that each country has followed. Secondly, the dissertation explores what the determinants of switching might be. Building on the concepts of exit and voice by Hirschman, I develop several hypotheses about how party switching is influenced by cost considerations that are situated either at the party level (costs of voice) or at the institutional level (costs of exit). Thirdly, the thesis introduces and studies the distinction between individual and collective forms of party switching, which have rarely been analysed by previous works on the topic. The results of the empirical chapters suggest that the overall level of party switching is mainly determined by party characteristics. In particular switching seems to be related to partiesâĂŹ ideological placement. Moreover, defections are trigged by low institutionalisation (measured both at party level and at the systemic level). In contrast, the role of institutions is very limited, except when I distinguish between the two modalities of switching. Specifically, parliamentary form of government contributes to collective switching as do electoral systems with high candidate visibility. Taken together, these findings contribute not only to shed light on the conditions that make switching more likely to occur, but also to the broader literature on party decline/adaptation and political personalization.