For some time NMR and ESR have been applied to the study of biological materials. Two somewhat distinct applications have developed: as a means of detecting a given complex and following it through a reaction, and as a source of information on the electronic state and structure of a material. Mo¨ssbauer spectrometry has recently been applied to biological studies, principally in the latter mode. Unfortunately, the method may be used only on atoms with suitable nuclear properties. Work has been reported on hemoglobin, myoglobin, various cytochromes, ferritin, ferridoxin, and xanthine oxidase, all of which contain iron. Hemoglobin has been the most intensively studied. The magnetic compounds (e.g., the azide, fluoride, nitric oxide) are sufficiently dilute that reasonably well‐resolved hyperfine spectra are seen at low temperatures. Many are consistent with ESR data and one (the azide) can be predicted from thegvalues. Recent measurements in applied magnetic fields confirm the original interpretations. The nitric oxide spectrum provides information on the spread of the unpaired NO spin onto the iron ion. Some spectra are not yet understood, with Mo¨ssbauer and ESR spectra indicating different spin states. A number of interesting problems relating to these, to spin‐spin interaction, and to other cooperative iron‐iron effects remain to be investigated.