In electronic equipment, the life, reliability, and operating characteristics of component parts (tubes, resistors, capacitors, motors, etc.) to a large extent are determined by temperature conditions within the parts. In practical cases, these internal conditions are best represented by one or more temperatures on the surface of each part. In special cases, with precautions, even large items such as pressurized equipment may be regarded as component parts, and their internal temperature conditions may be related to case temperatures. There is great need to be able to predict the surface temperatures of component parts without performing an endless series of tests, the results of which are ambiguous in many instances. The analysis presented is limited to prediction of surface temperatures of component parts which are cooled primarily by forced convection. In this case, conductive and radiative cooling would be relatively small. This limitation is not a major one, however, because it is expected that most electronic equipment, especially airborne equipment, will be forced-convection cooled. The heat dissipated by component parts by means other than convection will often appear at some other surface and be removed by con- vection.