Explaining EU climate unity: actors, interests and institutions.Two questions have been addressed in this study: first, to what extent have the EU countries appeared as a single entity in the climate change negotiations?, and second, what have been the main factors influencing the ability of the EU in this respect? The short answer to the empirical question is that the EU countries to a relatively high, and indeed increasing, degree have managed to appear as a unified entity in international climate politics. In the negotiations of the Climate Convention, adopted in 1992, individual member states did to a large extent `replace' the EU as the most important actor at the international level. In particular, different opinions about how to respond the US position made it difficult to the EU to hold a unified position throughout the negotiations. In the negotiations of the Kyoto Protocol, however, the EU did negotiate as a single entity. Prior the Kyoto Conference, held in December 1997, a formal negotiation mandate was agreed at the EU level. This enabled the leading Troika to negotiate on behalf of the member states, and the EU managed to speak with a single voice throughout the Conference.Three theoretical perspectives have been used to analyse the EU unity in the climate negotiations. The intergovernmental perspective has defined the national level of analysis. National economic interests as well as broader exogenous factors have been identified as independent variables in this perspective. The neo-functional perspective has focused on variables at the European level, with the role of the Commission and possible spillovers from other areas of European integration as independent variables. And finally, the external forces perspective has given attention to the international negotiation context in general, and the effect of the US as EU major external `opponent' in particular.A main conclusion is that the intergovernmental perspective has proven to go quite far in explaining the degree of EU unity in the climate negotiations. As long as international climate politics remains an issue area with restricted formal EU competence, the analysis of the EU as a climate policy actor should probably have the basic national preferences of key member states as its starting point. Nevertheless, also factors like the wider role of the EU as an actor in international affairs, the `external pressure' from the US and the Dutch Presidency's role in the internal burden sharing negotiations have influenced EU's ability to develop and maintain common positions in the negotiations. Thus, if variables at the EU level, or key aspects of the international context, is left out of the study, the possibility of reaching a comprehensive understanding of the subject will be significantly reduced.