This thesis constructs an explanation of transnational party politics, using the Party of European Socialists (PES) as a case study to test the relevance of existing theoretical work on the meaning, significance, and prospects for realising transnational party politics in the EU. Its originality can be found in three aspects. Firstly, rather than dismiss the PES a priori as merely a loose co-ordinating body, as much of the sceptic literature does, it takes seriously the claims of the transnationalists, especially Simon Hix and Robert Ladrech, that the PES had developed into something like a policy-seeking party. It operates from the assumption that a policy-seeking party's main goal is to influence the outcome of public policy, rather than the more traditional goals of vote maximisation or office seeking. Secondly, by subjecting the PES to careful scrutiny in two specific policy areas, those of employment and environmental policy, it takes the transnationalists' claims further by testing their validity by examining the PES's ability to construct policy for and influence the policy outcomes of the 1996-7 Intergovernmental Conference. Finally, it concludes that the PES was able to play a role in co-ordinating policy amongst the member parties, thereby challenging the views of the sceptic strand of the literature on transnational parties. However, its analysis of events, based upon an evaluation of the internal dynamics of PES policymaking between 1995 and 1997, suggests that Hix and Ladrech have exaggerated the power of the PES and underestimated the strength of the factors that limit its effectiveness. It argues that domestic policy imperatives and ideological differences between the member parties hindered the development of a true policy-seeking party, thereby advancing our knowledge of transnational party politics.