5. Conclusion Operations research has been one of the principal contributors to the enormous growth that the air transport sector has experienced during the past 50 years In the best tradition of OR, the development of models and of solutions has been motivated by issues and problems encountered in practice and has led, in several instances, to insights of a general nature and to important methodological advances in the OR field at large. At this point, OR models and algorithms are diffused throughout the sector and constitute an integral part of the standard practices of airlines, airports, and ATM service providers In view of the numerous challenges that it currently faces, it is safe to expect a continuing central role for OR in the air transport sector's future. As indicated in this paper, there are many promising topics for future research in each of the areas examined. At the most fundamental level, and in general terms, the frontiers can be summarized as follows: Relaxing the boundaries between the successive stages of aircraft and crew schedule planning, so that schedule design, fleet assignment, aircraft main tenance routing, and crew scheduling might eventually be performed in an integrated way, rather than solved sequentially as interrelated, but distinct subproblems Including pricing decisions in revenue management, instead of treating fares and fare classes as fixed, externally specified inputs Developing fast decision support tools that increase the safety and efficiency of transport operations by taking advantage of the massive, real-time data flows in an increasingly "info-centric" aviation infrastructure.