Over twenty years have passed since the insurrection in Kosovo which ended with the introduction of the North Atlantic Treaty Organization (NATO), in June 1999. The establishment of the State of Kosovo has raised multiple questions and challenged public international law doctrines in different ways. The validity of unilateral State succession in the post-colonial era, the international legal personality of international intergovernmental organizations, and the exercise of sovereign rights by other entities than States are among the questions that the international community has had to contend with. The present essay elaborates the aviation law-related aspects of these challenges, more precisely the reopening of the upper airspace of Kosovo, with a special focus on the legal solutions of the Implementing Agreement Between Hungary and the International Security Force in Kosovo (KFOR) (Implementing Agreement Between the Government of Hungary and International Security Force in Kosovo (KFOR) for the Provision of Air Navigation Services (ANS) and Other Relevant Activities in the Designated Airspace over Kosovo as promulgated by Act No. CCXLVIII of 2013 on Promulgation of the Implementing Agreement Between the Government of Hungary and International Security Force in Kosovo (KFOR) for the Provision of ANS and Other Relevant Activities in the Designated Airspace over Kosovo.). airspace, sovereignty, NATO, Kosovo, Hungary