This study examines processes of transnational alliance-building among political parties in the European Union (EU). It is associated with theories of transnational relations, neofunctionalist integration theory and domestic politics approaches to international co-operation and integration. The overall aim of the study is to contribute to the understanding and growth of knowledge of transnational party alliances, thereby expanding the field of party research. The author attempts to fulfil this aim through an in-depth analysis of the hard-won alliance between Conservatives and Christian Democrats in the European Parliament (EP), entered into by the British and Danish Conservatives in May 1992 and renewed after the 1994 European elections. What factors promote, respectively impede, the formation and evolution of a transnational party alliance such as this? A differentiation is made between three categories of process mechanisms by the causal roles they play, namely opportunities, motives and constraints. Factors are addressed under each category. A number of constraining factors on transnational alliance-building among political parties are traced and analysed. The domestic arenas of party politics are brought into the analysis to help us understand alliances in the European arenas. National parties appear to be anxious about their ideological identities and their autonomy, in the sense of freedom of action. Despite the constraints, political parties prove to be keen on entering into transnational alliances for different motives. They strengthen their capacity for action by building transnational channels for access and influence. By striking alliances in the light of threats posed by countervailing forces, they maximise their parliamentary influence. Tracing and analysing opportunities for such alliances, attention is paid to catalysts related to leadership changes, institutional changes and changes in the international environment. The analysis shows that the enhanced powers of the EP have had a major catalytic effect on transnational alliance-building among political parties in the EU. Patterns of informal integration are thereby established in response to changes in formal institutions.