The thesis studies the impact of the EU on security sector reforms (SSR) in the Western Balkans with an aim to investigate recent changes in the security dynamics of the region. The study covers the period from 1991, which marks the break-up of Yugoslavia, to 2013, the year when the first Western Balkan state joined the EU. It investigates changes in the security situation in the Western Balkans on both regional and state levels. While the region as a whole has become much more secure and peaceful, compared to the 1990s, its separate states display varying degrees of stabilisation and development. By comparing the EU’s involvement in SSR in Croatia, Serbia and BiH, which in the studied period achieved a full membership, candidate status and potential candidate status respectively, the thesis aims to determine if these differences can be linked to EU policies. The work argues that between 1991 and 2013 the character of security interdependence between the Western Balkan states changed from negative to positive, and that the EU was one of the key actors behind this change. To explain this transformation, it uses Regional Security Complex Theory (RSCT), that provides a comprehensive framework for analysing regional developments from the security point of view. The dissertation contributes to the study of the EU as a security actor and indicates the possibility of new applications of RSCT by examining the EU’s engagement with security sector reform and the evolution of security dynamics in the Western Balkans as the result of this engagement. It also extends the understanding of the effects and limitations of Europeanisation in states recovering from war and conflict.