The chemicals featured in this volume are at the top of "List of Substances Discharged into the Environment as a Result of Human Activities and Considered Hazardous to Sensitive Species of Natural Resources." The List was prepared over the period 1985-2003 by environmental specialists of the U.S. Department of the Interior. The metals, metalloids, organics. and radioactive substances chosen originated in wastes from agricultural, industrial, military, domestic, mining, and municipal sources. Some of these compounds were selected for inclusion because they had no known biological function and their presence in tissues is associated with adverse effects on growth, development, reproduction, and survival itself. Some have been incorporated into powerful biocides to control pestiferous organisms and. inadvertently, impact-desirable species of nontarget organisms. Others are highly prized by society, but the environmental consequences of extraction and refining them has adversely impacted habitats of plants and wildlife, sometimes for more than a hundred years. Several are essential to normal metabolism; however, insufficiency as well as excesses may be fatal. Most occur in a variety of chemical forms, some of which are comparatively benign and others extremely toxic. For each chemical or group of chemicals, basic information is presented on its sources, uses, properties, concentrations in living organisms, lethal and sublethal effects, identification of research opportunities, and proposed criteria to protect human health and natural resources. It is emphasized that all proposed criteria listed were recommended by local, regional, national, and international regulatory agencies, as well as knowledgeable university and industrial researchers. In general, regulatory agencies are required to periodically update all criteria incorporating the most recent scientific findings. Unfortunately, criteria -unlike legislatively mandated standards - are not legally binding, although in certain extraordinary cases, such as massive discharge of a chemical to the biosphere, regulatory agencies are known to impose financial and other penalties. Ultimately, as chemical risk assessment predictions based on suitable databases become increasingly reliable, standards will be established for individual chemicals, together with adequate funds for enforcement, and stipulated penalties for violators. This single volume compendium will provide a ready reference to professionals and students concerned with ecotoxicological aspects of numerous chemical wastes.