The contamination of groundwater in the United States is a challenging problem. It has been estimated that 300,000-400,000 contaminated waste sites may account for 750 billion dollars of remediation over the next three decades (National Academy of Sciences, 1994). The successful treatment of contaminated groundwater is a further challenge. Conventional treatment methods, such as pump and treat systems, have been shown to be somewhat ineffective. Selection of a groundwater treatment technology is a crucial and often costly proposition. Emerging groundwater treatment technologies may provide effective, lower-cost alternatives. It is important to fully understand all aspects of any innovative technology. This guidance document was developed to address the regulatory requirements of permeable reactive barriers (PRB) and try to achieve a consensus on requirements. It should prove useful to regulators, stakeholders and technology implementers. The document is divided into sections dealing with site characterization, modeling, permitting, construction, monitoring, waste management, maintenance, closure, health and safety and stakeholder concerns. From a regulatory perspective, the most important sections of the document are most likely the permitting, monitoring, and closure sections. Appendix B provides examples and current applications where PRBs have been, or will be, installed. Specifics regarding the design and installation of PRBs are not covered within this document, however the reference to documents that provide this information is included within the introduction. Site characterization is a critical step in order to deploy a PRB. A complete understanding of the site geology, hydrogeology, and geochemistry, as well as the contaminant profile, is necessary.