E.p.r. is a spectroscopic technique that probes the environment of a paramagnetic centre by defining tlie size and shape of the magnetic moment produced by the unpaired electron. and by characterizing any magnetic fields which might be produced by the parent molecule in the vicinity of the paramagnetic centre. In the context of our interests, these unpaired electrons are found either in selected transition metal ions or in certain organic free radicals. My talk will focus on the e.p.r. of biochemically interesting transition metal ions, for only these paramagnets exhibit a great variety of magnetic nionients. Organic radicals by contrast have identical magnetic moments (to within la). Notable examples of e.p.r.-active metalloproteins are listed in Table 1 and members of this Table will be discussed in detail by other contributors to this Colloquium. As is the case with most forms of spectroscopy, e.p.r. measurements of paramagnetic metalloproteins ;ire made with one of two objectives in mind. Conceptually the simplest. but by no means insignificant, is the ability of e.p.r. to provide a quantitative measure of the amount of paramagnetic species present in one's sample. In this instance. e.p.r. plays the same role as absorbance measurements in many other areas of biochemical research. The particular virtue of e.p.r. is that. for most systems, tlie intensity of the e.p.1. signal is independent of the chemical species so that a sample can be quantified without a priori knowledge of an 'extinction coefficient'. Such measurements occupy a large part of the e.p.r. method; iti general. they d o n o t require any special knowledge of the e.p.r. process (see Note 1 . Appendix 3 ) . The second objective is to obtain insights into the structural domain i t i which the paramagnet resides. and t o characterize the local environment of the paramagnet by probing tlic interaction of local electric and magnetic fields with the unpaired electron( s ) of the transition metal ion. This latter utilization of e.p.r. requires substantiallj, more understanding of tlie e.p.r. phenomenon than d o intensity measurements and the objective of this review is