In 2014, the RAND corporation, one of the leading American think tanks in security and defence, organised a wargame simulating a Russian invasion on the Baltic countries. According to this simulation, those countries could be under Russian control within 36 hours (Shlapack and Johnson, 2016). This study “made waves in Washington and other NATO capitals” and is said to have been behind NATO’s decision to undertake one of the biggest deployment of troops in Europe in a generation (Overhaus 2019: 19). This begs the question of what role external security experts have in shaping major NATO decisions. It does also question how external experts access the alliance, as well as with what discourses and recommendations. In order to explore those questions, this paper proposes a first assessment of how think tankers access the alliance and what are the main recommendations provided. While, at first, this may appear as a straightforward inquiry that would bring about simple answers, the lack of institutionalized interactions with external actors or official calls for experts has led to a rather ad hoc and fluid relation between NATO actors and think tankers. Furthermore, the heterogeneity of actors within NATO (i.e. military, diplomats and bureaucratic staffers), their corresponding professional cultures, and missions as well as their relative autonomy have shaped different approaches and expectations towards experts. By building on the literature on sociology of expertise, this paper intends to explore the interactions between think tankers and NATO, focusing separately on the bureaucratic, the diplomatic and the military part of the alliance.