The European Union - EU - came into being in the early 1950s with the aim of avoiding the rival clashes that had previously taken place in Europe. The starting point was a collaboration around the coal and steel industry, but the collaboration has spread to other areas. Likewise, the number of member countries has also increased, from initially consisting of 6 member countries to today consisting of 27. EU membership is preceded by various requirements that must be met for membership to be current. The accession criteria for EU membership, also known as the Copenhagen criteria, contain several requirements, of which the requirement of the fight against corruption is found among these. The criteria are a form of power exercised by the EU and this form of power, known as soft power, is widely used. Through the exercise of soft power, more countries are attracted to want to take part in the community that the EU constitutes, which leads to Europeanization and the enlargement of the EU. The well-known neo-functionalist theory of spillover states that integration leads to changes in the integrated countries. By carrying out a qualitative theory-testing case study regarding the spillover effects on the case of Croatia, the theory's validity is revealed. Does the EU-membership lead to changes in the integrated countries, and specifically regarding the fight against corruption? The research problem is important from a political science perspective and thus relevant to the research, and this work has therefore investigated this. The result shows that Croatia has made clear progress in their work in the fight against corruption, but one can also see difficulties in adopting EU policies. This is aconstant ongoing process that the country has to deal with.