The nature of d-transition metal centres in proteins is considered in several respects. The results of some recent protein crystallographic studies of enzymes are described; each enzyme requires a d-transition metal atom (or atoms) for activity. The advantages of combining protein crystallography with spectroscopic studies, in particular extended X-ray absorption fine structure (EXAFS) to provide accurate interatomic distances, are discussed. Also, situations where, to date, EXAFS has been the only source of structural information are considered. The use of bond valence sum analysis and a distortional theorem to inspect the details of the geometry of a metal binding site in a protein obtained by EXAFS analysis and/or protein crystallography are advocated.