Regional political parties and the European Union: Case study of Spain Abstract The diploma thesis "Regional political parties and the European Union: Case study of Spain" focuses on the area of the regional parties on the stage of the European Parliament, a subject which has not been analysed thoroughly in the past. Special emphasis is placed on the analysis of this phenomenon on the case of the Spanish historical regions (Catalonia, Basque Country, Galicia) which are a recognizable proof of the remarkable institutional strengthening in the framework of a regionalized unitary state. The methodology of triangulation was used for the analysis of this issue as the data of the diverse type (i.e. electoral results, party's material etc.) have been put together through different methods (as a qualitative and quantitative content analysis, deductive and comparative approaches) with the aim to reveal the complete character of the issue and to reach the objective results as possible. The study focuses on the analysis of the regional parties on the European level through their cooperation in the European federations of the political parties as well as the framework of the political groups in the European Parliament. The emphasis is also placed on the analysis of the own work of the members of the European Parliament...