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Selective enzymatic reduction is a biocatalytic reaction in which an oxidoreductase transfers reducing equivalents to one functional group, regioisomer, or prochiral face preferentially over competing possibilities. Ketoreductases and alcohol dehydrogenases commonly deliver hydride from NADH or NADPH to a carbonyl carbon while an active-site residue protonates oxygen, producing a stereodefined alcohol ^[1][2]. Ene reductases, imine reductases, and other reductases act on different acceptors but retain the defining combination of enzyme turnover and chemo-, regio-, or stereoselection. The protein binding pocket positions the substrate relative to the cofactor, and cofactor regeneration restores the reduced nicotinamide required for repeated catalytic cycles. The concept excludes stoichiometric chemical reductions, nonselective metabolic electron transfer, and enzymatic oxidations conducted in the reverse direction. It differs from metal-catalyzed asymmetric hydrogenation by using a macromolecular active site and usually a nicotinamide or flavin cofactor rather than molecular hydrogen at a metal center.