This chapter provides an overview of current airspace operations, introduces primary air traffic control (ATC) and traffic flow management (TFM) activities, and reviews issues facing the current system. Air traffic controllers are responsible for separating aircraft operating within the region, or sector, of airspace under their control. Regional TFM is based on improved aircraft demand, airspace capacity, and weather intent information. Surface operations in the United States are a shared responsibility between airlines controlling the ramp (nonmovement) area and the US Federal Aviation Administration Air Traffic Control Tower (ATCT), which controls traffic on taxiways and runways (or movement area). Surface operation research is investigating new technologies and concepts to increase airport capacity by enhancing the flexibility and efficiency of surface operations. The chapter describes new concepts in air traffic management and the key challenges in designing the future system. It also describes the implementation of the advanced concepts in NextGen.