Microwaves, such as those used in cooking and processing food, are part of the broad spectrum of electromagnetic radiation which includes radio waves, microwaves, infrared radiation, visible light, ultra-violet radiation, x-rays and Gamma rays. Electromagnetic radiation has a dual nature, it is both wave-like and particle-like. An understanding of this dual nature of electromagnetic radiation is necessary for an understanding of the processes of emission, transmission and absorption of microwaves, which is in turn necessary for understanding the processes and phenomena which are important in the use of microwave radiation as a source of energy for heating and food processing. The properties of electromagnetic waves and the processes of emission. transmission and absorption are described and some effects in microwave-heating applications are discussed.