This paper uses a Delphi approach to analyse policy issues related to the environmental aspects of the Single European Sky (SES). SES aims to organize the airspace uniformly, with air traffic control areas based on operational efficiency rather than national borders. One of the main aspects of SES is performance evaluation in four areas namely cost efficiency, capacity, safety and environment. This paper focuses on the latter using a two-round Delphi approach. The sample consists of thirty experts from airlines, Air Navigation Service Providers (ANSPs), Civil Aviation Authorities, Individual experts/consultant/academics, Governmental institutions, EUROCONTROL and the International Air Transport Association (IATA). The Delphi analysis is also supported qualitatively by five interviews with senior experts from the European Commission, EUROCONTROL and IATA. Among others, delays in implementation and the slow progress of the policy scheme are highlighted. Policy recommendations are also provided in the paper to effectively deal with the identified problems.