The paper is in two parts. The first deals with potted or encapsulated circuits and the second with printed wiring and, to some extent, with printed circuits.The present status of potted-circuit constructions and materials is considered, together with the evaluated properties of recently introduced resin systems which have increased heat resistance and more stable electrical properties. The mechanism of failure and remedies re discussed.Developments of printed wiring and printed circuits are compared, and details given of currently available techniques. Materials for both forms of construction are discussed in detail and their physical properties evaluated over a wide range of temperatures and climatic conditions. New low-loss materials are compared with established ones in respect of dielectric properties when wet or dry.There is a shortage of ‘Service’-quality components for printed wiring. Some guidance is given to designers of such components.