Scholars, policy-makers, and pundits have long debated the impact of the North Atlantic Treaty Organization on East European democratization. This article argues that NATO has been an essential instrument of democratic transition and consolidation because it has a) guaranteed security, a fundamental precondition of democratization; b) exerted a positive general influence on political processes; and c) urged and helped shape specific policies in the military-security domain. After explaining the East European states’ interest in gaining full membership in the Alliance, the article analyzes the debates preceding the 1997 and 2002 enlargement decisions and the ways in which 9/11 has changed NATO’s expansion.