Water and water use in southern Nevada is an important issue. The scarcity of water resources for both human and biologic communities often leads to intense competition for both surface and ground waters. Anthropogenic and climate change impacts on scarce water resources need to be understood to assess human and ecosystem health for southern Nevada. Chapter 3 outlines the state of the knowledge for hydrology in southern Nevada − ground water, surface water, and water quality − and provides recommendations for filling knowledge gaps. Information is presented that will help land managers develop strategies to achieve Sub-goal 1.3 in the SNAP Science Research Strategy, which is to “Restore and sustain proper function of southern Nevada’s Watersheds and Landscapes” (see table 1.1).