In living subjects electromagnetic signals are generated which can be measured electrically with electrodes and normal amplifiers or magnetically, by means of SQUID-magnetometers. The former technique is called EEG (electro-encephalography), the latter MEG (magneto-encephalography). Since the electromagnetic field patterns are dependent on physiological processes inside the body, a study of the electromagnetic field can help to understand these physiological processes. Some theoretical problems which are posed by such a study are considered. The way in which the electromagnetic field is generated is discussed. At a microscopic level, there are different physical mechanisms responsible for the generation of the electromagnetic field. Inside the brain, there are the synaptic interactions of neurons which produce the EEG and MEG. Cardiac potentials and cardiac magnetic induction are generated by the synchronous polarization of cardiac muscle cells.