Ground water is the Nation's principal reserve of fresh water and represents much of its potential future water supply. Ground water on National Forest System (NFS) lands is a major contributor to flow in many streams and rivers and has a strong influence on the health and diversity of plant and animal species in forests, grasslands, riparian areas, lakes, wetlands, and cave systems. It also provides drinking water to hundreds of communities. Demands for safe drinking water and requirements to maintain healthy ecosystems are increasing, and complex social and scientific questions have arisen about how to assess and manage the water resources on NFS lands. This technical guide was developed to help address these issues. Today, many of the concerns about ground water resources on or adjacent to public land involve questions about depletion of ground water storage, reductions in streamflow, potential loss of ground water-dependent ecosystems, land subsidence, saltwater intrusion, and changes in ground water quality. The effects of many human activities on ground water resources and on the broader environment need to be clearly understood in order to properly manage these systems. Throughout this technical guide, we emphasize that development, disruption, or contamination of ground water resources has consequences for hydrological systems and related environmental systems. Ground water and surface water are interconnected and interdependent in almost all ecosystems. Ground water plays significant roles in sustaining the flow, chemistry, and temperature of streams, lakes, springs, wetlands, and cave systems in many settings, while surface waters provide recharge to ground water in other settings. Ground water has a major influence on rock weathering, streambank erosion, and the headward progression of stream channels. Pumping of ground water can reduce river flows, lower lake levels, and reduce or eliminate discharges to wetlands and springs. It also can influence the sustainability of drinking-water supplies and maintenance of critical ground water-dependent habitats. Increasingly, attention is being placed on how to manage ground water (and surface-water) resources on public lands in a sustainable manner. The potential for ground water resources to become contaminated from anthropogenic as well as natural sources is being scientifically assessed. Each ground water system and development situation is unique and requires a specific analysis to draw appropriate conclusions. This technical guide begins by reviewing the legislative and policy framework, and the issues related to ground water inventory, monitoring, contamination, and development. Individual sections then focus on key concepts, principles and methods for managing ground water resources. Relevant special topics, case studies, and field examples are highlighted throughout the text. Additional information on some topics can be found in the appendixes. This technical guide provides guidance for implementing the U.S. Department of Agriculture (USDA) Forest Service national ground water policy. It describes hydrological, geological, and ecological concepts, as well as the managerial responsibilities that must be considered to ensure the wise and sustainable use of ground water resources on NFS lands. This document is one part of a four-part information system on ground water management on the national forests and grasslands. The other three parts are (1) Forest Service policy on ground water (Forest Service Manuals [FSM] 2543 and 2880); (2) a Forest Service sourcebook on State ground water laws, regulations, and case law for all 43 States with NFS land; and (3) a ground water inventory and monitoring technical guide. When complete, the four parts will provide line officers and technical specialists at all field levels with the science, policy, and legal framework for Forest Service ground water-resource management.