Microcellular plastics, otherwise known as microcellular foam, is a form of manufactured plastic fabricated to contain billions of tiny bubbles less than 50 microns wide (typically 0.1–100 micrometers). It is formed by dissolving gas under high pressure into various polymers, relying on the phenomenon of thermodynamic instability to cause the uniform arrangement of the gas bubbles, otherwise known as nucleation. Its main purpose was to reduce material usage while maintaining valuable mechanical properties. the density of the finished product is determined by the gas used. Depending on the gas, the foam's density can be between 5% and 99% of the pre-processed plastic. Design parameters, focused on the foam's final form and the molding process afterward, include the type of die or mold to be used, as well as the dimensions of the bubbles, or cells, that classify the material as a foam. Since the cells' size is close to the wavelength of light, to the casual observer the foam retains the appearance of a solid, light-colored plastic.

Recent developments at the University of Washington have produced nanocellular foams with cells in the 20-100 nanometer range. At Indian Institute of Technology Delhi, technologies are being developed to fabricate high quality microcellular foams.