This paper outlines the U.S. EPA's findings to date regarding existing state ground-water quality monitoring activities. First, the major components of an ambient monitoring program are described. Second, data representing eight state ambient ground water-quality monitoring program budgets, and well installation and laboratory analysis component costs are summarized. third, to assist state and local programs in evaluating monitoring costs, this paper describes an algorithm for estimating the costs associated with new or expanded ambient ground water quality monitoring programs. Finally, the paper concludes with a discussion of those factors that could further reduce the costs associated with ambient ground water quality monitoring. These data are provided to give federal, state, and local programs guidance on the likely range of costs associated with existing and expanded ambient ground water-quality monitoring programs.