This paper presents a framework to assess Air Traffic Management (ATM) performance in relation to flight efficiency. The main philosophy behind the approach presented in this paper is to quantify the quality of the service delivered by an Air Navigation Service Provider (ANSP) to an airline in terms of meeting commonly agreed-upon objectives. The definition is, therefore, aligned with the future paradigm of Trajectory Based Operations, where achieving the trajectory agreed between the ANSP and the airspace user becomes the focus. A staged approach to flight efficiency assessment is proposed to quantify the quality of the ANSP’s service in terms of both “facilitating what has been agreed” and “improving what can be agreed.” The framework promotes the development of more consistent efficiency performance metrics between ANSPs, by defining clear definitions for assessment references. Application of the framework is illustrated with several examples using the Airservices Dalí trajectory modeler.