NATO’s adaptation in the post-cold war era has included a growing role in responding to cyber attacks. This article explores the definitional and theoretical problems in analysing the emerging cyber security field and seeks to provide an outline of the main strategic challenges that cyber attacks pose for NATO. The article evaluates the institutional structures and processes put in place by NATO to confront cyber threats and moves on to explore the extent of transatlantic unity around NATO’s emerging doctrine for cyber defence. It is argued that NATO’s response to cyber security is rooted in its prior adaptation to the changing security environment of the 1990s and the threat from international terrorism post 9-11. More recently, cyber attacks from state and state-sponsored actors have given impetus to NATO’s emerging cyber security doctrine. The article finds that despite recent controversies over cyber surveillance, there is a considerable degree of unity within the alliance on the main cyber security issues. As a multilateral security organisation with a strong institutional foundation, NATO is also relatively well placed to respond to rapidly evolving cyber threats.