SUPERSTITION AND QUACKERYdominated experimentation and speculation on interaction of magnetic fields with living systems for well over a thousand years. More critical inquiry is of relatively recent origin. There is still controversy about a variety of findings, but sound observations and applications of magnetic interactions to biology and medicine are beginning to emerge. Thus, for instance, constant magnetic fields have been shown to affect plant growth and animal development, alternating magnetic fields have been used to stimulate nerves and to evoke visual sensations, and activity of nerves and muscles has been shown to be accompanied by transient magnetic fields. Most advanced is the development of magnetic methods of measurement. These methods include electromagnetic determination of blood flow and magnetic resonance spectrometry of biological molecules.