In situ biorestoration, where applicable, is indicated as a potentially very cost-effective and environmentally acceptable remediation technology. Many contaminants in solution in ground water as well as vapors in the unsaturated zone can be completely degraded or transformed into new compounds by naturally occurring indigenous microbial populations. The key to successful remediation is a thorough understanding of the hydrogeologic and geochemical characteristics of the contaminated area. The need today is for methodology to determine when natural biorestoration is occurring, the stage the restoration process is in, whether enhancement of the process is possible or desirable, and what will happen if natural processes are allowed to run their course.