Nature uses the properties of open-shell transition metal ions to carry out a variety of functions associated with vital life processes. Binuclear iron centers, in particular, are intriguing structural motifs present in proteins and enzymes such as methane monooxigenase, which converts methane to methanol, and which display (anti)ferromagnetic ordering. We have developed computational methods based on spin density functional theory (SDFT) to elucidate structures, spin-dependent processes, and physico-chemical properties of metal sites in proteins. In addition, the active sites of some metalloproteins undergo spin-forbidden biochemical reactions which are important but poorly understood processes during which the total electronic spin is not conserved. Examples include the binding reactions of paramagnetic ligands, such as dioxygen and nitric oxide, to heme and non-heme iron proteins. Studying the physical origin of spin-forbidden reactions is important for understanding the reactivity and function of many metalloproteins. I will discuss computational studies which elucidate the roles of spin-orbit coupling and evaluate activation barriers associated with spin-forbidden ligand-binding reactions. Our results have established important correlations between the electronic structure, geometry, spectroscopic data, and biochemical function of heme and non-heme iron proteins.