This article examines the changing transatlantic security architecture since the end of the Cold War. It argues that the absence of a unifying military threat and the subsequent broadening of the notion of security from states to societies has led to the increasing differentiation of security policy arrangements since the beginning of the 1990s. Not only have international institutions proliferated since the end of bipolarity, private actors - such as non-governmental organizations and private security companies - have gained considerable influence. Since these features are not fully grasped by traditional models in security studies, the article suggests that a new theoretical perspective might be required if we are to understand the emerging security system. It proposes that such a perspective can be based on the concept of `security governance', which describes the development from the centralized security system of the Cold War era to the increasingly fragmented and complex security structures of today.