This is the first of two articles about the physics of microwave ovens. This article deals with the generation of microwaves in the oven and includes the operation of the magnetrons, waveguides and standing waves in resonant cavities. It then considers the absorption of microwaves by foods, discussing the dielectric relaxation of water, penetration depths of electromagnetic waves in matter and, in considering the possible chemical changes during the microwave heating, multi-photon ionization or dissociation.