An area of intense scientific and practical interest is the biogeochemical and microbial processes determining the success of natural attenuation, biostimulation and/or bioaugmentation treatments for organic contaminants in groundwater. Recent studies in this area have focused on the reductive dechlorination of chlorinated solvents, the degradation of the fuel additive methyl tert-butyl ether, and the removal of long-term hydrocarbon contamination. These studies have been facilitated by the use of stable isotope analysis to demonstrate in situ bioremediation and push-pull tests, in which isotopes are injected into aquifers and then quickly retrieved and analyzed, to measure in situ activity. Molecular tools such as quantitative PCR, the detection of mRNA expression, and numerous DNA fingerprinting methods have also proved valuable, being employed to identify and sometimes quantify environmentally important organisms or changes in communities. Methods to track bacteria and tools to characterize bacterial attachment properties have also offered insight into bacterial transport in situ.