Massive sulfide and gold deposits are the two most problematic metallic mineral-deposit types in the Appalachian region from an environmental perspective. The environmental impacts of abandoned mines developed from massive sulfide deposits result from the formation of metal-laden acid drainage and from the presence of fine-grained, metal-rich mine wastes. Suites of problematic metals associated with these deposits differ with the type of massive sulfide deposit; in the Appalachian region, these are generally of either Kuroko or Besshi type. Mine drainage is mostly a threat to aquatic ecosystems, but metals also can contaminate local drinking water supplies. Abandoned gold mines are of environmental concern because of the mercury used in the gold-extraction process, which poses a threat to human health due to its ability to bioaccumulate in the foodweb, especially in aquatic systems.