The European Parliament (EP) now acts as an equal co-legislator with the Council of Ministers in adopting many policies that affect 500 million European citizens on a daily basis. However, the parliamentary legislative organisation is under-researched despite its profound consequences for EU policies and policy-making. Addressing this gap, this thesis studies the internal setup and legislative impact of the EP committees. Drawing on congressional literature, I confront distributive, informational and partisan theoretical approaches to answer the research questions of this project, namely whether and why the EP committees and their legislative output are dominated by preference-outlying legislators with special interests, experts serving the informational needs of the plenary, or loyal members of the working majority party group (coalition). Statistical analyses of committee assignments, allocation of legislative tasks, and adoption of committee reports in plenary are conducted using data on the 6th European Parliament (2004-2009). They are complemented with evidence from semi-structured interviews. The results show that legislators’ special interests and expertise account for the formally regulated assignment to committees depending on the predominant character of their legislative output (distributive or regulatory). In contrast, party group affiliation and loyalty shape the allocation of important legislative tasks in committees, owing to the informal allocation process. Furthermore, committee reports are more successful on the floor if drafted by rapporteurs from the working majority party group - perhaps a natural consequence of the EP open amendment rule. Thus, the parliamentary legislative output is ultimately controlled by the working majority party group and not committees. The congressional rationales fail to account for committees’ legislative influence when an informal early agreement is reached with the Council of Ministers. This occurs increasingly often, rendering decision-making in committees largely obsolete. The observed regularities are used to advance the literature on legislative organisation by identifying conditions under which each of the main congressional rationales can explain committee setup and influence, namely: 1) the policy areas a committee covers; 2) the parliamentary rules regulating committee-party and committee-plenary relationships; and 3) the balance of power and mode of negotiation between the legislative chambers. More substantively, the EP committees are not conducive for pursuing particularistic policies. Instead, they promote left-right party politics. This has important implications for EU legislative politics, interest representation, legitimacy, and more generally the EU democratic deficit.