Microwave technology has become important in preparatory techniques for microscopy in many different ways. This paper discusses various aspects of the physics of microwaves. It gives some theoretical background to understand the practical procedures. Some peculiarities in the optics of microwaves are pointed out, and the practical implications in particular of choosing the size and shape of samples and containers are discussed. Diffusion rates and chemical-reaction rates increase exponentially with temperature, so that precise temperature control is essential in most histochemical procedures. Such control is complicated by localized heating of the system, and of temperature sensors themselves, which may occur as a result of microwave irradiation.