Environmental performance is a significant aspect of aviation. Apart from reducing the emissions of the air navigation service provider, it is important to consider the airspace structure. The Single European Sky (SES) aims to organise 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: cost efficiency, capacity, safety, and environment. In the key performance area of environmental, targets aiming at climate change mitigation have been set for flight efficiency and airspace structure. This chapter discusses the various measures and considers some challenges for improved environmental performance.