The rate of occurrence of air traffic control operational errors is recognized as an important measure of the safety performance of the National Airspace System. As the volume of air traffic increases and new automated air traffic control decision support tools are introduced, there is a concern not only to increase the level of safety in the system by reducing the occurrence of operational errors but also to ensure that efforts to increase the capacity of the system do not adversely affect safety. Recent trends in air traffic control operational errors in the United States are analyzed. Factors that have been found to influence the rate of occurrence of these errors are discussed, and the relationship between the severity of operational errors in an en route environment and the traffic characteristics at the time are examined. The limitations of the information currently available to support the analysis of operational errors are described. Finally, the data requirements are addressed for future research to better understand the causal factors affecting the rate of occurrence of operational errors and to develop strategies to enhance the safety of the National Airspace System.