Recent work on the absorption of microwave radiation in the frequency range 10⁹ c/s to 3 × 10¹⁰ c/s is reviewed, with particular reference to materials of biological interest. The relative importance of ionic conduction, dipolar relaxation, particularly of water molecules, and resonance is discussed briefly. Conventional methods for dielectric constant and loss measurements at microwave frequencies can be adapted for use with biological materials and a number of such modifications is described. The results of investigations into the dielectric properties of water, the most abundant biological material, proteins and a variety of body tissues are outlined and discussed in terms of established dielectric theory. High power microwave generators have been used to study tissue heating, the heating pattern comparing favourably with predictions from the radiation propagation constants and the thermal properties of the materials. A few experiments on the effects of microwaves on animal tumours and on viruses are described.