Sealing of boreholes and underground excavations has not received much engineering attention until fairly recently. The growing awareness of and sensitivity to environmental concerns of the technical community as well as of the public at large has resulted in an increasing recognition of the fact that these geological penetrations may have an environmental impact. The issue of possible contamination resulting from migration along boreholes, adits, shafts or tunnels unquestionably has been raised most forcefully with­ in the context of nuclear waste disposal. Several nuclear waste disposal programs, notably the Civilian and the Defence programs of the US Department of Energy, the US Nuclear Regulatory Commission and the Canadian and Swedish radioactive waste disposal programs have conducted major research efforts aimed at developing adequate seal designs for penet­ rations in host rock formations for high-level nuclear waste repositories. While a considerable data base has been gathered over the last two decades or so with regard to the performance of seals, most of the information is presented in research reports and widely scattered papers in journals and proceedings of conferences. Hence, the materials are not readily accessible to potential users such as designers, contractors or regulators who are not familiar with nuclear waste disposal programs.