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The asymmetric reduction of C¼C bonds is a powerful tool to generate chirality as it leads to the creation of (up to two) stereogenic centers. Flavoproteins from the "Old Yellow Enzyme" (OYE) family catalyze the NAD(P)H-dependent asymmetric bioreduction of alkenes bearing an electron-withdrawing (activating) group in a strict trans-fashion (Scheme 3.1.1). Since these oxygen-stable enzymes display a broad substrate spectrum and high stereoselectivities, their use for synthetic applications represents a highly competitive alternative to transition metal catalysts (homogeneous catalysis 1,2 ) and small organic molecules (organocatalysis 3 ). [4][5][6] This methodology has proven to be highly efficient and is applicable to the following types of activating group: 5,7-14 i. Only C¼C bonds conjugated to an electron-withdrawing group are reduced, electronically non-activated ('isolated') double bonds are not accepted. With conjugated 1,3-and cumulated 1,2-dienes, only the a,b bond is selectively reduced. Carbonyl conjugated alkynes yield the corresponding (E)-alkenes. 15