Distributed Air/Ground Traffic Management (DAG-TM) is a detailed instantiation of the Free Flight concept, formulated to serve as a foundation for NASA’s long-term research on advanced air traffic management concepts. Potentially revolutionary methods of air traffic management will be necessary to achieve the mature state of Free Flight. DAG-TM is a human-centered operational concept that seeks to minimize the impact of air traffic management constraints by leveraging new procedures and technological innovations in automation aids, information sharing, and supporting technologies. Under the DAG-TM paradigm of operations, flight crews, air traffic service providers (controllers), and aeronautical operational control personnel (flight planners/dispatchers) utilize distributed decision-making to maximize user flexibility and system throughput. This paper introduces the DAG-TM concept and provides a detailed description of the en route concept elements; the main purpose is to disseminate the DAG-TM concepts to the air traffic management community, and to motivate further exploration.