Subsurface engineered barriers have been used to isolate hazardous wastes from contact, precipitation, surface water, and groundwater. The purpose of this report is to provide the U.S. Environmental Protection Agency’s (EPA) waste programs with a national retrospective analysis of barrier field performance, as well as information that may be useful in developing guidance on the use and evaluation of barrier systems. The report focuses on vertical barriers; evaluation of caps was a secondary objective. The overall approach to the report was to assemble existing performance monitoring results from a number of sites, and examine those results in light of remedial performance objectives and factors that may influence performance, specifically, design, construction quality assurance/construction quality control (CQA/CQC), types of monitoring programs, and operation and maintenance (O&M) activities. This performance evaluation report describes the performance of subsurface engineered barriers at each of 36 sites. The report discusses the performance evaluation process, including the site identification and selection process, and the availability of information upon which to base judgment of the performance of existing barriers and presents findings and conclusions, including observed similarities or trends among sites. This document is available on the Internet at www.clu-in.com. A limited number or Appendices (document No. EPA-542-R-98-005a) are available from the EPA National Center for Publications and Information (NCEPI) at 1-800-490-9198.