PEDRO J. J. ALVAREZ, PhD, is the George R. Brown Professor of Civil and Environmental Engineering at Rice University and the 2005–2006 President of the Association of Environmental Engineering and Science Professors (AEESP). His teaching covers the applications and implications of biological treatment processes, and his research focuses on bioremediation of contaminated aquifers, fate and transport of hazardous substances, and treatment of contaminated soil, water, and wastewater. WALTER A. ILLMAN, PhD, is an Assistant Professor of Hydrology at the University of Iowa. His teaching covers contaminant hydrogeology and vadose zone hydrology, as well as engineering geology. His research focuses on subsurface water flow and contaminant transport in both porous and fractured geologic media.