Groundwater is a basic resource for humans and natural ecosystems and is one of the nation's most important natural resources. Groundwater is pumped from wells to supply drinking water to about 130 million U.S. residents and is used in all 50 states. About 40 percent of the nation's public water supply and much of the water used for irrigation is provided by groundwater. Despite the importance of groundwater as one of our most precious natural resources, an organized, effective program to provide an ongoing assessment of the nation's groundwater resources does not exist. With encouragement from the U.S. Congress, the USGS is planning for a new program of regional and national scale assessment of U.S. groundwater resources, thus helping bring new order to its various groundwater resources-related activities. The Survey's senior scientists requested advice in regard to the design of such a program. In response, the committee undertook this study in support of developing an improved program relevant to regional and national assessment of groundwater resources. This report is a product of the Committee on USGS Water Resources Research, which provides consensus advice on scientific, research, and programmatic issues to the Water Resources Division (WRD) of the U.S. Geological Survey (USGS). The committee is one of the groups that work under the auspices of the Water Science and Technology Board of the National Research Council (NRC). The committee considers a variety of topics that are important scientifically and programmatically to the USGS and the nation, and it issues reports when appropriate. This report concerns the work of the WRD in science and technology relevant to assessments of groundwater resources on regional and national scales. The USGS has been conducting scientific activity relevant to groundwater resources for over 100 years and, as summarized in Appendix A, today groundwater-related work occurs throughout the WRD.