Diiron proteins represent a diverse class of structures involved in the binding and activation of oxygen. This review explores the simple structural features underlying the common metal-ion-binding and oxygen-binding properties of these proteins. The backbone geometries of their active sites are formed by four-helix bundles, which may be parameterized to within approximately 1 A root mean square deviation. Such parametric models are excellent starting points for investigating how asymmetric deviations from an idealized geometry influence the functional properties of the metal ion centers. These idealized models also provide attractive frameworks for de novo protein design.