This paper aims to assess the progress made towards the implementation of a Single European Sky (SES). It examines the program’s four main key performance indicators - environmental impact, safety, cost efficiency and capacity - and depicts in more detail the sequence of changes in each problematic area, thereby providing a better visualization of the main milestones and the issues hindering proposed reforms. The paper concludes that the overall process of reforming European airspace within SES has been slow and delayed, partly due to a lack of cooperation and commitment from air navigation service providers and states to deliver the information needed and apply desired amendments. The paper recommends several actions, which could improve the SES performance, including among others the implementation of incentives to meet the program’s objectives, as well as penalties for lack of cooperation, and the introduction of a common en-route charging rate at the European level.