This thesis aims to clarify how the Single market policy and the Social policy of the European Union has changed since the Treaty of Rome (1957) and what opportunities and/or constraints there might be for integrating gender mainstreaming within the policies.  In order to determine how the policy development affects the prospects of integrating gender mainstreaming, a case study supplemented with the method of content analysis is carried out. Furthermore, the thesis adopts a historical institutionalist perspective. Thereby, policy change is operationalised with the help of Peter Hall ́s theory of the three orders of change. It is therefore assumed that the concept of path dependency will explain how policy changes opens up opportunities for or constrain the integration of gender mainstreaming in the policy-making process.  The main findings show that the Single market policy mostly changed in accordance with the first order of change and that the Social policy mostly changed in accordance with the second order of change. These findings imply that a paradigm shift of the fundamental ideas and objectives of the policies have not occurred since the signing of the Treaty of Rome which complicates future integration of gender mainstreaming in the policies.