Transition metal ions are essential for the specific function of many biomolecules, like transport and activation of small molecules, electron transfer, etc. Their active sites contain (i) single paramagnetic or diamagnctic metal ions, or (ii) clusters of spin-coupled paramagnetic centers. Examples are for (i) hemoglobin or rubredoxin with one Fe, and for (ii) ferredoxins with [2Fe-2S] and [4Fe-4S] clusters, cytochrome c oxidasc containing Fe and Cu, or peroxidases with metal-radical pairs. Here, we present the results of our Mössbauer and EPR investigation of irondioxygen interaction in horse liver alcohol dehydrogenase (HLADH), of ironradical interaction in synthetic analogues of heme peroxidases and of iron-iron double exchange interaction in a synthetic ferric/ferrous mixed valence complex. We hope to demonstrate that the understanding of spin coupling can yield valuable insight in the relation between electronic structure and function of biological iron complexes.