Steroid compounds can be ranked among the most widely marketed products from the pharmaceutical industry. Highly specific reactions are required to produce functionalized compounds with therapeutic use and commercial value. The complexity of steroid molecules renders the use of biocatalysts particularly interesting, due to the high regio- and stereo-selectivity of the reactions to be performed. These characteristics, together with the mild conditions required for bioconversions, led to the development of high yield biological production processes. which are more environmentally friendly than their chemical synthesis counterparts, a feature that is presently a major concern of industrialists. Although some of these bioconversions are well-established, efforts are ongoing in order to increase the efficiency of the existing processes as well as to identify new potentially useful bioconversions. In this work an overview of recent developments on the use of microorganisms for steroid production is presented. (C) 2003 Elsevier Science Inc. All rights reserved.