Publisher Summary The chapter provides some specific examples of stereoselective biotransformations to prepare chiral intermediates required for the synthesis of pharmaceutical drugs. The search for selective enzyme inhibitors and receptor agonists or antagonists is one of the keys for target-oriented research in the pharmaceutical industry. It is now known that in many cases only one stereoisomer of a drug substance is required for efficacy and the other stereoisomer is either inactive or exhibits considerably reduced activity. New drugs for the clinic should be homochiral to avoid the possibility of unnecessary side effects due to an undesirable stereoisomer. Chiral drug intermediates can be prepared by different routes. One is to obtain them from naturally occurring chiral synthons mainly produced by fermentation processes. Second is to carry out resolution of racemic compounds. This can be achieved by preferential crystallization of stereoisomers or diastereoisomers and by kinetic resolution of racemic compounds by chemical or biocatalytic methods. Chiral synthons can also be prepared by asymmetric synthesis by either chemical or biocatalytic processes using microbial cells or enzymes derived therefrom. The advantages of microbial or enzyme-catalyzed reactions over chemical reactions are that they are stereoselective and that they can be carried out at ambient temperature and atmospheric pressure. This minimizes problems of isomerization, racemization, epimerization, and rearrangement that generally occur during chemical processes. Microbial cells or enzymes derived therefrom can be immobilized and reused during many cycles. The chapter provides some specific examples of stereoselective biotransformations to prepare chiral intermediates required for the synthesis of pharmaceutical drugs.