The European Parliament is a unique legislative body. It is a supranational legislature with directly elected members. The policy authority of the EP has increased dramatically as the European level legislation has grown both in depth and scope. This thesis addresses the question of what political pressures most influence members of the EP when they are voting on legislation. The members are directly elected from national constituencies as members of national political parties. Once elected to the EP, members sit according to ideology in transnational party groups. Potentially then, the national constituency, the national political party, and transnational party groups all pressure MEN to vote a particular way on given pieces of legislation. I propose a principal-agent theory that assesses the relative strength of these competing pressures. While there is reason to expect each 'principal' to pressure MEPs, I argue that it is necessary to take the policy content into account. Specifically, I hypothesize that distributive policies are more likely to bring national pressures to bear, while voting on regulatory policies is more likely to be governed by ideological considerations. I test these hypotheses using a large sample of roll-call votes chosen specifically for their policy content during the period 1984--1994.