This chapter analyses the on-going development of North Atlantic Treaty Organisation (NATO) in the context of the changing international security agenda. It explores how NATO underwent a fundamental transformation in the 1990s turning itself from a traditional collective defence alliance into a Europe-wide security organisation engaged in peacekeeping and co-operation with its former enemies to the east. The chapter shows that - despite the organisation's transformation during the 1990s - the longer-term future of NATO remains uncertain. A major challenge that NATO faced in the 1990s was the conflict in former Yugoslavia. In the early 1990s there was reluctance within NATO to transform the Alliance from a collective defence organisation to a wider peacekeeping/enforcement body. Despite NATO's transformation during the 1990s, the long-term future of the Alliance remains uncertain. Since the mid-1990s some analysts have argued that if NATO is to remain relevant to the security needs of both the US and Europe the Alliance must adopt a global role.