The Edwards aquifer system is one of the great karstic aquifer systems of the world. It supplies water for more than 2 million people and for agricultural, municipal, industrial, and recreational uses. The Edwards (Balcones Fault Zone) Aquifer in the San Antonio, Texas, area was the first to be designated a sole source aquifer by the Environmental Protection Agency in 1975. The Edwards Aquifer also hosts unique groundwater, cave, and spring ecosystems. This 27-chapter memoir reviews the current state of knowledge, current and emerging challenges to wise use of the aquifer system, and some of the technologies that must be adopted to address these challenges.