The proliferation of environmental regulations and subsequent concerns for soil and groundwater pollution has helped the field of hydrogeology evolve from its geologic roots and move from its early hydraulic affiliations with water supply and geotechnical concerns into a more complex discipline. The study of water as an environmental science has drawn froma number of disciplines, including advancedmathematics and computermodeling towater well drilling. As practicing hydrogeologists, we have found ourselves referring to amyriad of sources during our professional activities for consistent definitions. Further, in this search, we have also discovered an evolution in the definition of terms. In response, we have compiled an extensive list of terms typically encountered in the field of hydrogeology. We have endeavored to be as complete as possible without becoming ‘‘a series.’’ This necessitates excluding various subjectwords that, although related, are not directly needed by the practicing hydrogeologist or is better covered ina separate book.