One measure of the effectiveness or safety of the Air Traffic Control system is the occurrence of an operational error. The purpose of this study was to determine contributing factors and conditions surrounding the occurrence of an operational error. A longitudinal analysis of controller workload at the time of the error revealed some time related changes. While most errors occurred under moderate workloads (40-50%), since 1965 the trend has been for a smaller percentage of errors to occur under heavy workload conditions, with an increase in the percentage Involving light workloads. While the precise determination of causal factors is not possible with the current system, emphasis should be placed on the importance of attention, planning, judgment, coordination, and communications. Some suggestions are made concerning the gathering of additional information during the review process to assist in the overall analysis of an operational error.