SummarySince the separationsrequired for safety are a major factor in airport and airway capacity, and since new developments in ATC and avionics dilute the value of accumulated experience, there is a pressing need to formulate a universally accepted rationale for the establishment of aircraft separation standards. This paper examines the consequences of the point of view that separations must be adequate for ATC to provide effective, essentially redundant, protection against blunders or failures aboard the aircraft.The most conservative objective for separation standards is that they keep any two aircraft from reaching a position, velocity configuration from which it is possible for a collision to occur before effective ATC intervention can be accomplished. In implementing this philosophy one must consider position and velocity errors, computation and communication lags, as well as pilot and aircraft response delays. Further, however, the development requires a complete hazard detection and resolution strategy. Thus in addition to specifying separation standards, one must, in the process, also specify the primary data required for hazard assessment, the computational algorithms required for hazard determination, the complement of corrective commands, and an algorithm for the selection of the most effective command. These considerations define position and velocity thresholds which represent the last chance for effective intervention by air traffic control. Within these boundaries, air traffic control has the capability of providing assured safety. If the boundaries are penetrated, however, there are combinations of errors and delays which allow a possible unpreventable collision. If the existence of this possibility cannot be tolerated, the boundaries derived specify the minimum separation tolerable to ATC.