The rate of growth in air transportation demand of 5-6% per year is unlikely to be significantly influenced by constrained runway capacities at primary airports within industrialized countries. Using a recently developed model that predicts airline operational responses to airport capacity constraints, Evans and Schafer (in this issue)show that airline would adjust operations within a constrained flight network in such a way as to avoid airports with high delays. In particular, airlines would reroute their flights towards less congested secondary airports,and ,in addition,increase the use of larger aircraft. However, the latter effect is small as airlines compete for market share by increasing their flight frequencies,an effect that naturally favors the use of smaller aircraft.While most system-wide implications for operations seem to be manageable, local impacts at congested hub-airports may be significant in terms of arrival delay,energy use, and emissions.