This book outlines the infrastructure and methods needed to measure, monitor, and regulate the concentration of mercury present in the environment. It proposes a set of indicators to use as a measure of changing mercury concentrations in air, water, soil, and aquatic biota. Next, it offers a monitoring strategy and guidance for determining if concentration levels are changing systematically over time. The book explains how additional monitoring strategies can relate observed changes in mercury concentrations to regulatory controls on mercury emissions. The final chapter provides an integrated framework for establishing a national-scale program to monitor mercury concentrations in the environment.