The UN and NATO have been jointly engaged in a range of conflicts in the post-Cold War era. Studies of these organizations, however, have largely overlooked the institutional interplay between their Secretaries-General. After brief reviews of the relationship between the UN and NATO and the leadership role that a Secretary-General can provide, this article examines the political relationship between Kofi Annan and Javier Solana across three stages of NATO's 1999 Operation Allied Force in Kosovo. The findings show the important roles played and coordinated effort supplied by the Secretaries-General. This provides new perspectives on UN-NATO institutional coordination and has important implications for considering the relative security roles to be played by the UN and NATO in the future. KEYWORDS: UN Secretary-General, Kofi Annan, NATO Secretary-General, Javier Solana, Operation Allied Force. IN AN AGE OF SHIFTING SECURITY CONCERNS AND APPROACHES MARKED by the end of the Cold War, the UN and NATO have faced a series of challenges. In the process these two organizations, which primarily operated as distinctly separate entities during the Cold War, have needed to closely consider their relationship as they address common conflicts and continue to face a range of potential joint security concerns. Institutional interaction and coordination have largely evolved in an ad hoc manner in response to these situations, but recent efforts to establish a framework for cooperation demonstrate how the UN-NATO institutional relationship remains an important and pressing issue.