Published by the American Geophysical Union as part of the Geophysical Monograph Series, Volume 122. Among the current problems that hydrogeologists face, perhaps there is none as challenging as the characterization of fractured rock. Within hydrogeological systems, general issues concerning groundwater flow and environmental remediation cannot be resolved in any practical manner prior to investigating the nature and vagaries of the fracture networks themselves. Comparable difficulties arise when developing economic programs for the exploitation of oil, gas, and geothermal reservoirs in fractured rock. Equal, if not greater, difficulties have commanded our attention relatively recently in regard to the storing of spent fuel generated by nuclear power plants. For example, if we are to isolate spent nuclear fuel in underground rock systems, we must construct a repository to protect the biosphere from contamination by radioactivity while subjecting the total rock system to a significant thermal field for many thousands of years. Predicting the behavior of a waste repository under such conditions, especially in fractured rock, is a formidable task.