The most basic question this chapter tries to address is the current and future projected discrepancy between water supply and water demand within the United States and what practices will need to be implemented to ensure sustainable water resources in the future. Surface water and groundwater both contribute to the national supply and are used in different proportions that vary by state. Trends in past usage and analysis of present issues create a picture of what water resources will be available in the future. This chapter summarizes the state of water resources in the United States and addresses surface water, groundwater, quality of water resources, and water use and sustainable reuse methods. An application section provides three Illustrative Examples related to the general subject of water resources in the United States.