The application of electromagnetic induction to the measurement of blood flow and blood velocity in vivo is reviewed, The electronic requirements are discussed and the associated problems are surveyed. A brief account is given of Bevir's virtual current theory and its application to flow and velocity measuring devices. The devices at present known to be in use or to be under development are reviewed under the following headings: invasive local magnetic field devices which include cannular and perivascular flowmeters, also intravascular velometers and flowmeters, invasive extracorporeal field perivascular and intravascular flowmeters; and noninvasive systems which either utilise skin electrodes and a separate extracorporeal magnet or comprise an integral extracorporeal flowmeter. There is a brief discussion of the present state of development.