This paper outlines a human error identification (HEI) technique called TRACEr--technique for the retrospective and predictive analysis of cognitive errors in air traffic control (ATC). The paper firstly considers the need for an HEI tool in ATC, and key requirements for the technique are noted. The technique, which comprises a number of inter-related taxonomies, based around a simple cognitive framework, is then described. A study concerning a real-world application of TRACEr is outlined-the evaluation of several options for reduced separation minima in unregulated UK airspace. In this study, TRACEr was used predictively and retrospectively, looking forward to pre-empt potential problems and looking back to learn from experience. The paper concludes that TRACEr is a valuable aid to design, development and operations in UK ATC, and has indeed been used as a basis for further applications in ATC both in Europe and the USA.