initial planning period of about five years, CDM was fielded for the first time in 1998 in connection with the FAA’s Ground Delay Programs (GDPs), which go into effect whenever long air traffic delays are anticipated at an airport due to poor weather or other reasons, thus often necessitating ground holding. CDM marks a truly fundamental innovation in the ATM system, possibly the most important one in at least 30 years. The three main elements on which it is based are: (a) a dedicated data communications network (“CDMnet”), which facilitates the continuous exchange of infor mation between the FAA and the airlines (plus any other CDM participants) about the current and near-future states of the ATM system; (b) the use of a common database and a common set of software tools by all CDM participants; and (c) the partial decentralization of decision making. With respect to (c), it is the FAA’s responsibility to forecast the capacity that will be available at each part of the ATM system during the relevant time horizon, as well as to allocate this capacity among the individual airlines and the other ATM system users. And it is the responsibility of each individual airline to decide how it will use its allocated share of capacity at each part of the system. This is a somewhat simplistic description of what, in practice, is a complicated process that employs several types of distributed decision-making techniques, such as rationing by schedule (RBS) and schedule compression—see Wambsganss (1996) and Vossen et al. (2003) for details