Stereoselective catalysis comprises catalytic transformations in which one stereoisomer is formed preferentially over other constitutionally identical stereoisomers. Selectivity may be enantioselective, favoring one enantiomer, or diastereoselective, favoring one diastereomer; it is quantified by enantiomeric ratio, enantiomeric excess, diastereomeric ratio, or related stereochemical measures. A chiral catalyst creates diastereomeric transition states from an achiral or prochiral substrate and lowers their activation barriers unequally, whereas an achiral catalyst may still produce substrate-controlled diastereoselection ^[1]. Catalytic asymmetric hydrogenation exemplifies enantiotopic-face discrimination by a chiral metal complex ^[2]. The defining feature is kinetic discrimination within a catalytic cycle, not merely isolation of a single stereoisomer after a nonselective reaction. Stereoselective catalysis is therefore broader than enantioselective catalysis, but narrower than stereospecific reaction: stereospecificity maps different substrate stereoisomers to different products, while stereoselectivity concerns unequal product formation from a common substrate.