This chapter deals with two aspects of the interaction of magnetic ‹elds to biological tissue: (i) the use of rapidly changing magnetic ‹elds (caused by rapidly changing electrical currents in coils) to stimulate excitable tissue, and (ii) the measurement of the tiny magnetic ‹elds associated with naturally occurring current loops within the body. There is a reciprocal relationship between the two phenomena: the same result is obtained if we treat a particular small current loop within the body as a source and we measure a magnetic ‹eld (or induced current in a loop of wire on the surface, which is called magnetometry) as when we have a loop of current on the surface of the body as a source (magnetic stimulation) and we measure the induced current at the same location in the body where the source loop was previously. Of course, in the ‹rst situation, the magnetic ‹elds concerned are around 10-12 those associated with the second. In both cases, the currents concerned are those associated with neural activity (spontaneous or perhaps evoked in the ‹rst case and suf‹ciently strong to evoke responses in the second case).