The aim with this thesis is to find out how one can understand the exceptions not to introduce the euro as the currency of Denmark and the United Kingdom, as well as their attitude to European integration. It has been implemented in a comparative case study using theories in which the case has been the central focus of the investigation. The theories being used is rational actor model that emphasizes rational decision-making and self-interest. In comparison to that theory has a historical institutional perspective been used, which emphasizes path-dependency and critical events. The focus of the study is the time when the countries became members of the European Economic Community in 1973 to the Maastricht Treaty in 1993 where these exceptions not to introduce the euro as the currency was ratified. In the analysis section, one can see portions of both theories to a varying degree. The conclusions of the study is that both countries show a skeptical attitude to European integration and that the exceptions to not introduce the euro as a currency is based on this critical view of moving power to centralized institutions.