Two materials being used extensively as cryogenic insulations are the expanded plastics polyurethane (PU) and polystyrene (PS). These cellular plastics are excellent thermal insulators with measured values of apparent thermal conductivity at a mean temperature of 112 K being 0.013 to 0.016 W/m·K. The effect of aging and conditions of aging are critical factors in determining the ultimate conductivity. The major component of the overall conductivity, gas conductivity, is affected by permeation of gases through the cell walls. The reasonably high strength-to-density ratio for PU and PS at room temperature, 8 to 9 kPa/kg/m3 compared with ∼50 for aluminum alloys and ∼25 for stainless steels, allows these insulations to be self-supporting and to lend structural support to the system. The thermal, elastic, and mechanical properties are functions of several nonindependent parameters such as density, cell size, composition, and method of processing. The properties of these expanded plastics are also anisotropic. The large variations found in existing data reflect these complex dependencies. Expected behavior and experimentally determined data are discussed for the following properties: tensile, compressive, and shear strengths and the associated moduli, thermal conductivity, linear thermal expansion, specific heat, and thermal diffusivity. Temperatures below 300 K are considered.