Biodegradation as a component of natural attenuation of chlorinated solvents, is observed at most sites where groundwater has been impacted by these compounds. The critical issues frequently include: whether degradation proceeds to complete dechlorination; is the plume maintained short of the point of compliance/property line; how long will monitoring and evaluation be required, and what is the cost compared to other more “active” alternatives. Case studies include sites where degradation of TCE and PCE is complete, either through reductive dechlorination to ethene or through apparent sequential reductive dechlorination /oxidation pathways, and where the plume extent is in steady state or shrinking. Evaluation of whether to proceed with MNA alone or in combination with an active remedy including source control measures has been considered in each case. Enhanced reductive dechlorination was selected for one site based on life cycle costs. In each case source area control has been selected as a component of the remedy. The data presented supports observations and ideas presented by others that cis-DCE can be attenuated through either reductive dechlorination alone or through alternative mechanisms. The amount of electron donor and the time over which it has been available appears to be an important factor in determining which mechanistic pathway predominates.