The need and requirements for Elasto-Plastics materials for the encapsulation of pressure, temperature, and environmental sensitive devices is described. The properties of these materials that make them suitable to meet these requirements are discussed and the problems that have been associated with the use of other more rigid encapsulation systems is described. The applications of Elasto- Plastics materials as used at the Hawthorne Works of the Western Electric Company is described in some detail along with the proc essing requirements. These applications include transformers, glass enclosed switches, inductors, capacitors, coatings and adhesives. Materials that are used include polyurethane, RTV silastic, and flexible epoxy resins. The miniaturization of electronic devices has made feasible many applications that would have been too costly if large quantities of materials were required in the encapsulation.