The articles in this collection describe characterization and monitoring methods for measuring, tracking, and evaluating the biological degradation of environmental contaminants. Included are discussions of methods for monitoring petroleum hydrocarbon biodegradation in soils and evaluating field-scale treatability, shoreline bioremediation, biodegradation kinetics, stable isotope monitoring, toxicity of treatment waste products, continuous monitoring technologies, and substance gas tracers for vadose zone NAPL characterization.