A geomembrane is defined as a very low permeability synthetic or reinforced bituminous sheet placed in contact with soil. They are used extensively in transportation applications (tunnels), environmental applications (landfills, sand reservoirs) and geotechnical related applications (dams) in substitution or in addition to compacted clay, bentonite, concrete, steel and bituminous materials. They are used to contain or restrain gas and liquid movement. Geomembranes are manufactured in industrial plant and are polymeric and/or bituminous products shipped on site in rolls prior to being installed and seamed to obtain a continuous impermeable layer. Welding or adhesive techniques are used to bond the sheets together. In this book, in situ geomembranes are excluded. Historically, geomembranes were used in the 1930s while widespread adoption started in the 1940s. In 1987 more than 50 million m2 were installed worldwide and future growth of 20 to 25% per year is forecast for the next five years.