Heat is generated in a variety of dielectric or electrical insulating materials when subjected to radio and microwave frequencies. This phenomenon has been successfully exploited for over 40 years by industry in a wide range of applications. Radio frequency (RF) is used for about 90% of all dielectric heating applications, but the more recent development of microwave heating is becoming well established. The place of RF and microwaves in the electromagnetic spectrum and the effects of different frequencies on materials is discussed. Brief details of the various techniques of applying the RF or microwave energy to materials are given. Finally, some typical applications such as welding, baking drying and preheating are discussed.