17 beta-Reduction is one of key and widespread reactions of microbial steroid metabolism. It accompanies various steroid transformations by microorganisms and plays a role in the regulation of reducing equivalent pool and detoxication of exogenic steroids. The ability of microorganisms to reduce C-17-ketosteroids has been known for 60 years, but most progress in the biochemistry and biotechnology of the process has been recorded in the past two decades. The regulation of 17 beta-hydroxysteroid dehydrogenase activity is of importance for the selectivity of various steroid biotransformations. The potential application of microbial 17 beta-reduction and the reverse reaction -17 beta-oxidation - in the pharmaceutical industry for the production of 17 beta-hydroxy and 17-oxosteroids is shown. Microbial side chain cleavage of sterols in combination with 17 beta-reduction represents the shortest way to obtain testosterone in a single biotechnological operation thus replacing multistep chemical and/or combined biotech-chemical syntheses. This review highlights progress in the understanding of the biochemical basis of 17 beta-reduction and the biotechnological applications of the process. (c) 2004 Elsevier Ltd. All rights reserved.