Abstract Volcanic rocks are ubiquitous throughout the North American Plate, having formed along convergent plate margins, in rifts, and over hot spots. They range in age from earliest Precambrian to Recent and are exposed or are within easy reach of drilling over a relatively large area of the plate (See Plate 1). Volcanic rocks originate in the earth's upper mantle and lower crust and comprise a large variety of forms. The diversity of forms results in significant diversity of the occurrence, movement, and chemistry of ground water within them. This hydrogeologic variability is caused not only by different geodynamic emplacement and geologic processes but also by differences in hydrologic factors of rainfall, recharge flux, evaporation, topography, soil, vegetation cover, and other factors discussed in Lindholm and Vaccaro (this volume). Geologic controls on the occurrence, movement, and solute composition of ground water in volcanic rocks are largely a function of volume and degree of interconnection of pore space. Primary porosity and permeability of features such as lava tubes, flow breccia, clinkers, flowtop rubble, and shrinkage cracks far exceed those of fractures and joints due to regional tectonic stresses. Other rocks, such as ash-flow tuffs and sheet-flow basalts, may have very low primary permeability and porosity. Thus, processes (chemical, physical, or biologic) that create, enlarge, diminish, or eliminate pore spaces act in concert with the geologic controls and significantly effect the hydraulic characteristics of the rocks.