We have long understood and continue to improve our understanding of the geochemical conditions that favor biodegradation of petroleum hydrocarbons. Over the last several years our understanding of the conditions that are necessary for and/or promote reductive dechlorination of chlorinated hydrocarbons has improved substantially. Over this time in situ remediation technologies have proliferated. Many sites are being addressed with active remediation in conjunction with natural attenuation. For chlorinated solvents in particular, regulatory agencies, including USEPA, recognize the use of monitored natural attenuation but express a clear preference for its implementation, when appropriate, along with source control measures. It is thus beneficial to recognize that active remediation technologies alter aquifer condition in ways that may affect natural attenuation processes. We call attention to the direct and indirect effects on natural attenuation of several active remediation technologies. It is important to anticipate these effects when selecting and designing remediation strategies for impacted sites.