The accumulation of large amounts of ash from fossil fuel combustion for electric power plant generation is becoming a major environmental concern in the United States. Furthermore, stringent environmental regulations mandated by the Environmental Protection Agency through the Clean Air Act, Clean Water Act, Resource Conservation and Recovery Act, as well as state and local environmental regulations may result in even more ash production with subsequent contact with the environment. The concentrations of trace elements in coal residues are extremely variable and depend on the composition of the original coal, conditions during combustion, the efficiency of emission control devices, storage and handling ofbyproducts, and climate. The research papers in this book were presented as a part of the Sixth International Conference on the Biogeochemistry of Trace Elements held at the University of Guelph, Ontario, Canada, from July 29-August 2, 2001. The purpose of this corit'erence was to present current knowledge on the source, pathways, behavior and effects of trace elements in soils, waters, plants and animals.In addition, the book also includes invited research papers from scientists who have done significant research in the area of coal and coal combustion byproducts. All the research papers presented herein have been subjected to peer review.