Transition metal complexes are located at the active sites of a number of enzymes involved in intriguing biochemical reactions. These complexes can exhibit a wide variety of chemical reactivity due to the ease at which transition metals can adopt different coordination environments and oxidation states. Crystallography has been a powerful technique for examining the structure and conformational variability of complex biological metallocenters. In particular, the past ten years have provided a wealth of structural information and several surprises concerning the metallocenters at the active sites of nitrogenase, hydrogenase and carbon monoxide dehydrogenase/acetyl-coenzyme A synthase.