4PDH- This presentation details the historical and contemporary developments of the horizontal and vertical geodetic datums of the United States including: the Bessel reference system, U.S. Standard Datum, North American Datum, North American Datum of 1927, North American Datum of 1983, First, Second, Third and Fourth General Vertical Adjustments, the National Geodetic Vertical Datum of 1929 and the North American Vertical Datum of 1988. The program highlights changes in measurement and positioning technologies and their impact on the development of reference ellipsoids, geoid models and contemporary high accuracy reference frame enhancements. This session is designed to provide a foundation for other seminars that will focus on details, plans and programs of the 2022 reference system changes by the National Geodetic Survey.