Use atmospheric CO2 to produce chemical feedstocks and fuels. Mo & W-formate dehydrogenases (FDH) to catalyze CO2 reduction to formate. FDHs harbor a conserved active site with a terminal sulfo group plus one SeCys or Cys. FDHs catalysis proceed via a sulfur shift or a hydride transfer mechanism? FDHs are models to develop biocatalysts/synthetic catalysts to efficiently reduce CO2.