This AWWA revision to Manual M21 is the culmination of nearly three years of effort by members of the Groundwater Resources Committee. This edition has been written to provide the reader with a general understanding of the principles involved with groundwater, its movement and character, and the subsequent impact these characteristics have on the design, construction, and maintenance of groundwater well systems for water utilities. Among the major changes included in content from prior editions are the incorporation of the well construction standards formerly attached to Standard A100 Water Wells and a discussion in chapters 1 and 3 on sustainability of groundwater supplies in light of competition, lack of regulatory limitations, overdrafting, and climate changes. In addition, groundwater protection, planning, and evaluation efforts have evolved during the past 10 years, which have been updated in chapter 3. Modeling techniques have developed as well, to the point where many consumptive-use projects include a modeling exercise. New uses of groundwater, such as aquifer storage and recovery, and new legal issues with interbasin transfers have evolved as well.