This thesis presents a critique of the partisan control thesis, a common claim in the academic literature on the European Parliament that two partisan actors - domestic political parties and the European parliamentary groups (EP Groups) - influence how Members of the European Parliament (MEPs) approach representation. Specifically, it investigates whether domestic parties and EP Groups shape how MEPs think about and carry out representation, and what factors are linked to variation in the degree to which these partisan actors seek and achieve influence. Adopting a mixed-methods research design, this thesis analyses data from the 2010 EPRG survey of MEPs, and a body of original data gathered by conducting interviews with MEPs and officials. Three parties from Finland - KOK, the SDP, the PS - and three from the UK - the Conservatives, Labour, and UKIP - are selected as case studies. The MEPs examined are affiliated to one of four EP Groups, namely the EPP, the S&D, the ECR, and the EFD. This thesis finds that neither domestic political parties nor EP Groups exercise the degree of influence that the partisan control thesis suggests. Furthermore, it identifies that three factors are linked to the propensity of national parties to attempt to influence MEPs, and that a further three factors determine the desire and ability of EP Groups to influence MEPs. This thesis argues that although it is beneficial that MEPs are given the freedom by their parties to carry out their work according to their own judgment, the low levels of attention domestic parties pay to the activities of MEPs gives rise to concerns regarding the existence of an 'accountability deficit' in the EU. The pessimistic conclusion is that this deficit is unlikely to be addressed unless parties come to place greater value on goals that lie within the context of the EU's political system.