This thesis aims to discuss the Collective Security Treaty Organization (CSTO) and the sources of Russia&rsquo / s influence over its other member-states. It focuses on the origins of the CSTO and the development of security cooperation within the CSTO framework. The thesis argues that although the CSTO continues to be a Russia-centric regional security organization, Russia&rsquo / s influence over the other CSTO member states has been gradually limited throughout the 2000s due to the increasing diversity in the threat perception of the other CSTO member states and the increasing penetration of the other regional security organizations into the post- Soviet space. Following the Introduction chapter, the second chapter discusses the origins of security cooperation within the framework of the CIS. The third chapter analyzes the CSTO in terms of its structure and activities since its establishment in 2002. The next chapter outlines the transnational challenges to the security of the post-Soviet states and their threat perception, as well as the efforts to promote regional security by the regional actors. The fifth chapter analyzes the increasing involvement of other regional security organizations, such as SCO, OSCE, and NATO into the post-Soviet space. The concluding chapter discusses the main finding of the thesis.