This chapter discusses the context of the aviation system: aircraft, people and the tasks they perform, and the environments in which aviation is conducted. Airline aircraft range in age from brand-new to a few first-generation transports built during the early 1960s. The aircraft found in general aviation range from antique biplanes built in the 1920s up to modern, high-speed, pressurized single- and multiengine airplanes fully capable of operation under all weather conditions in the airspace system. The wide diversity of aircraft operating in the national airspace system presents pilots and controllers alike with serious problems. The psychomotor requirements are considerable, although accident experience suggests that few air carrier accidents involve failures in the "stick-and-rudder" aspects of piloting. The average air carrier pilot flies somewhat more than 500 hours per year, and both military and civil experience indicate that considerable exposure is necessary to remain fully proficient in the demanding tasks of flying complex aircraft.