The research project deals with the historical and comparative reconstruction of treaty change and the constitutional evolution in the European Union. The focus is on changes in the decision-making procedures and on the horizontal and vertical distribution of power in the eight cases of EU treaty change from 1957 to date. At the heart of the project is an analytical framework for the investigation of motives, processes and the evaluation of results with regard to possible problems of effectivity, integration and legitimacy. Along with the comparative analysis of the cases the interrelation of the eight processes shall be examined. Both, political science and law have so far mostly been concerned with the substantial content of the revised treaties while leaving aside the processes of treaty change. The project shall contribute to eliminate this research deficit.