The goal of this paper is to develop a conceptualised understanding of the PES, as a transnational European political party. In so doing it will draw up a framework via which we can actually evaluate a transnational political party. The central claim is that such an understanding rests upon the interdependent relationship of four party-streams (factions) and a realisation of each streams position vis-à-vis weak or strong institutionalisation. I will analyse the resonance of this claim by looking at the historical and contemporary development of the PES. This will encompass the following dimensions: the party's genesis and its continuing legacy and the views of the streams concerning the future development of the PES. [ABSTRACT FROM AUTHOR]