The Federal Aviation Administration (FAA) is currently upgrading the National Airspace System (NAS) to provide improved safety, increased system capacity and additional services to users. One of the major areas of the FAA's modernization efforts is in communication systems improvements, both ground based and air-ground communications. The FAA Aeronautical Data Link Program was established to develop, evaluate and implement air-ground digital data communications capability and the applications that will be transmitted over air-ground data links. The Mode S data link program developed by the FAA will provide the first data link service in the NAS. This paper provides a survey of the applications currently considered for implementation via data link, and shows how data link services will be implemented initially and in the longer term.