Numerous investigations over the years have illustrated the importance of physical methods, especially magnetic resonance, in the elucidation of the electronic ground states of 3d transition metal ion complexes. The results of these studies coupled with investigations on model compounds are being extended in very meaningful ways to provide electronic structural information in biological systems. In systems not readily amenable to investiga tion by single crystal X-ray diffraction these methods provide structural information as well. It was felt that a NATO Advanced Study Institute that brought together experts in theoretical aspects of magnetic resonance, experts in biology and experts in both areas would be extremely profitable for all parties attending the school. The enthusiastic response of the participants indicated that our objectives were accomplished to a high degree. We hope this pUblication of the proceedings will transmit, to some degree, the stimulating discussion of the Conference. The enormity of the area selected for this Conference can be appreciated by all. We the editors take full responsibility for the many omissions resulting from the imposed limitations of time and resources. Florence, July 1979 I. Bertini R. S. Drago PARTICIPANTS Andersen, J. P. Institute of Medical Biochemistry - University of Aarhus DK-8000 Aarhus C Denmark Andersson, I. Fachbereich 15. 2, Analytische und Biologische Chemie, Universitat des Saarlandes - D-6600 Saarbrucken 11 F. R. G. Basosi, R."