The rapidly increasing density of air traffic has created a demand for instrumentation to reduce the possibility of air catastrophes due to collision. The incidence of reported near-collisions is now estimated to be over 2000 per year in the United States, and represents a serious threat to the future growth of air travel. Since mid-1967, research has been conducted by NASA and the Research Triangle Institute to develop techniques for evaluation of collision warning systems and to determine the characteristics of the operational environment. Actual aircraft traffic data have been obtained by the FAA from the radar system at the Atlanta Airport. These data have been analyzed to determine encounter statistics which would result with various forms of warning criteria. This paper reviews the definition of several proposed warning criteria and presents the results of the statistical analyses of the data. Probabilities of encounter, encounter rates, and average encounter durations are determined for various definitions of the encounter.