Transition metal ions in biological systems are of interest in biology, biochemistry, chemistry, medicine, and physics. Scien­ tists with rather different viewpoints, employing many methods, have contributed to this area. A concise review of the current state of the field will, to some extent, reflect the special knowledge of the person writing it - in this case application of physical methods to the investigation of metal coordination. x­ ray diffraction is one of the most important of these methods, but a useful treatment of X-ray structure analysis would be com­ parable in size with and beyond the scope of the monograph. Many results of X-ray diffraction studies are, of course, presented. Electron paramagnetic resonance spectroscopy has played a major part in the rapid advance in knowledge of the electronic struc­ tures of transition metal ions in biological systems. More gener­ ally, measurements involving light, microwaves, and magnetic fields are capable of producing much new information, and the required instrumentation is available at most research institu­ tions. Therefore light absorption and paramagnetic resonance are treated in depth. The principles described in the latter discus­ sions are broadly applicable, for example to the promising tech­ niques of X-ray spectroscopy (utilizing synchrotron radiation) and lanthanide-perturbed, very high-resolution nuclear magnetic resonance spectroscopy.