This article presents an overview of ground water hydrology. Ground water occurs almost everywhere beneath the land surface and is an integral part of the hydrologic cycle. Basic concepts of the movement of water within porous media and differences in behavior between unconfined and confined aquifers are reviewed. A discussion of natural recharge and human influences on recharge leads to a section on ground water flow systems and their spatial and temporal scales. The importance of ground water budgets and the effects of pumping and climate on these budgets are discussed next. This is followed by a section on ground water quality, including contamination of ground water from various sources, effects of surface water/ground water interactions on water quality, and saltwater intrusion. The article concludes with a discussion of the principal types of data required for analysis of ground water systems and the role of modeling.