The paper discusses the role of the Organisation for Security and Cooperation in Europe (OSCE). Relations between international organisations are, as a rule, characterised by both cooperation and competition. Cooperation of the OSCE with NATO and the EU in Southeastern Europe reflects the changes that have taken place in recent years among the main security structures in Europe. The OSCE specificity is that all the 19 states members of NATO and the 15 states members of the European Union also belong to the OSCE. For this reason, a concept of labour division emerged several years ago among these three security structures. According to that concept NATO would deal with military aspects, the EU would tackle politico-economic issues and the OSCE would focus on social and humanitarian matters. As all 19 states members of NATO and the 15 states members of the European Union also belong to the OSCE such division could not function and has not functioned for many reasons. The OSCE's role in restoring stability is less spectacular than NATO military operations or EU economic assistance though it is no less essential. It promotes the building of democratic institutions and the shaping of security based on the respect for common values.