Résumé : This PhD dissertation analyses the processes used by political parties to select candidates for the European elections, making the unique institutional setting and electoral dynamics of the European Parliament a “litmus test” for party organisations. By applying an institutional design theory to the ways parties nominate their candidates for the EP, I can test a wide range of pre-existing postulates about parties’ behaviours, and can do so across many different countries (thus considering various electoral settings and socio-political arenas) while also accounting for the multi-level setting in which they operate. To this end, I thoroughly explore the intra-party “machinery” at work in the drafting of nominees. More specifically, this dissertation first provides a descriptive account of the main intra-party formal rules that govern the EP selection procedures from a comparative perspective. Concretely, this descriptive analysis serves to measure the relative power of individuals, party organs, and party levels, and to establish what imperatives (membership fees, endorsements, incompatibilities, quotas) are prescribed by the various parties. On that basis, I identify the patterns of selection rules put in place by the parties. I then explore the factors that condition the parties' choices of procedures by relating the aforementioned observations to a number of national- and EU-level characteristics. Later, I confront these rules with the parties’ informal practices during the actual process, and further trace an entire process from the rules' enactment to the choice of candidates. In doing so, I hope to contribute a small but important building block to the understanding of current political parties, while also speaking directly to those who are more generally interested in comparative politics and EU politics.