This paper presents a framework for the assessment of 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 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, as clear definitions exist for assessment references. Application of the framework was illustrated with several examples using the Airservices Dalí trajectory modeller.