There has been much effort to increase airspace capacity and one of the fundamental research problems is to measure the traffic complexity inside sectors. This paper proposes a new method for describing the air traffic complexity of a given traffic situation. We view the airspace as a closedloop input-output system and define air traffic complexity as "how difficult" a given traffic situation is, in terms of the control activity required to resolve it, in response to a change in "reference signal", that is the presence of a new aircraft entering the airspace. We present a "complexity map" that offers a graphical view of the complexity for a given traffic situation. We also discuss how to extract a scalar measure of air traffic complexity from the complexity map. We illustrate our methodology with a few examples.