This synthesis document will be of interest to materials, maintenance, and bridge engineers, and others responsible for protecting and maintaining portland cement concrete (PCC) roadway surfaces. It will be of special interest to materials research and testing officials, as well as to manufacturers who are concerned with developing and evaluating sealers for PCC highway facilities. The information in this Synthesis is limited to surface-applied liquid sealers that are primarily intended to retard the transmission of water and ions below the roadway surface. This report of the Transportation Research Board provides detailed information on generic concrete sealing materials that may be considered a primer on the topic, describing the various classes of sealers, their chemical characteristics, application to specific needs, and practice and performance characteristics. In addition, information is provided on evaluation and testing, application and costs, and the safety and environmental issues of concern with their use. Also included is a glossary of terms, extensive references, and an appendix that includes materials on a procedure for the estimation of service life and a model sealer specification.  The report for this topic can be purchased at https://www.trb.org/news/blurb_detail.asp?id=3348