The general aim of this paper is to describe how the European Union can affect and transfer their conditionality politics to the Western Balkan countries. A specific purpose of this study is to describe and explain what we in this paper consider to be a specific additional membership criterion, regional cooperation that has been especially designed for this region. The point of departure for this paper is Europeanization theory that will be used to study the European Union’s impact on non EU-members in Europe. The theoretical Europeanization mechanisms are taken from Heather Grabbes “Europeanization model”. A descriptive case study method together with a theory consuming method will be used. After the wars in former Yugoslavia the now independent countries started in their own pace to move closer the European Union. The problems the individual countries faced in the beginning of the 21^st century were much a consequence of the war. This lead the European Union to establish a special political and economical regional framework, where regional cooperation was emphasized. Regional cooperation developed into one of the most important elements in the EU´s relation with the countries and is made a specific requirement under the Stabilization- and association process. Implications this paper makes are that regional cooperation can be considered as a specific requirement the EU use in the Western Balkan countries membership process and that EU have a series of “influencing tools” which works to promote regional cooperation in the Western Balkans.