Despite dramatic changes in the international security environment that occurred following the end of the Cold War, key international security organizations (ISOs) have not only managed to persist but have done so by transforming profoundly. However, the existing literature has predominantly focused on the fact of their persistence, failing to scrutinize, systematically and theoretically, the crucial issue of their change. Bearing this gap in mind, this thesis seeks to explore in what way have key ISOs transformed after the Cold War and, in particular, why these transformations have taken the form they have. It does so by relying on insights from practice theory. Therefore, the thesis argues that ISOs are best understood as a constellation of "practices of talking" and "practices of doing". Drawing on Bruno Latour's notion of order making practices, it then proposes that "practices of talking" that unfold in their high-level panels should be considered as principal constitutive practices of these organizations due to possessing a higher ordering capacity. Put differently, they are practices through which state representatives and participating bureaucrats create, advance, juxtapose, and assemble, various types of claims, norms, rules, and principles so as to arrive at particular narratives of international (security) order in view of structuring, on that basis, relations and practices within and without ISOs. In addition to thus fashioning a particular "content" of the international order, they also delineate an ISO's agency with respect to it or, as termed in this thesis, an organizational intent. Based on this joining of a content of an order narrative and an organizational intent attached to it, they are then able to develop an ISO's specific institutional design and corresponding "practices of doing". In light of this conceptualization, the thesis proposes that ISOs transform via (re)negotiation of ordering links/logics in their order narratives (an inevitable consequence of their "communication" with external practices they seek to structure) and corresponding shifts thus introduced to notions of their own purpose/intent. In this regard, I suggest that ISO's transformation can be studied as a fluctuation between their intention to (1) build/construct a new international (security) order, (2) reinforce an order already existing in their environment; (3) merely reflect it; or (4) seek to transcend it because it is considered as excessively strained and thus undesirable. This theoretical and conceptual framework is than used for the analyses of the manner in which the OSCE, the UN, and NATO transformed following the end of the Cold War.