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This article is dedicated to a class of specialty materials that have come to be known as low-density microcellular materials, or LDMMs. LDMMs have been developed and studied over the last decade at several U.S. Department of Energy (DOE) National Laboratories. LDMMs are used in various high energy physics programs where they meet unusual and demanding specifications (see Table 1.1).To set the stage for introducing LDMMs, it is instructive to contrast them with more familiar cellular materials. Examples of commercially available low-density materials are plastic “blown cell” foams, such as polyurethane cushions and polystyrene coffee cups. Asymmetric membranes and filters, on the other hand, are representative of commercially available microcellular materials. The processes used to make these products generally are not suitable for making LDMMs, however, because they are limited by a tradeoff between density and cell size. That is, they yield low densities only at the expense of increased cell size, or small cell size at the expense of increased density. The challenge to DOE materials scientists, therefore, has been to develop new methods for making materials that simultaneously have low density and microcellular morphology, and also meet other application requirements (e.g., composition, homogeneity, size, and strength).LDMMs are used by the DOE laboratories because their low density and high homogeneity (implied from their microcellular morphology) make them ideal precursors or substrates for forming high-density gases or expanded plasmas.