A collection of microorganisms shows the ability to perform chemical transformations of xenobiotic compounds into products of commercial value. These processes, known as bioconversion, offer several technical, economical and environmental advantages in comparison to chemical synthesis, such as the selective formation of products with chemo-, regio- and enantioselectivity, the decrease on by-product formation, the use of mild reaction conditions and lower energy consumption.  Whole cell biocatalysts show a more stable enzymatic apparatus and can be more readily and inexpensively obtained in comparison to enzymes that have to be isolated and purified. Furthermore, the use of viable cells for redox reactions is greatly beneficial due to their ability to regenerate cofactors. Biotransformation based processes have also benefited from recombinant DNA technology, which has enabled the construction of tailor made cells overproducing a specific enzyme of interest. As biotransformations allow the selective production of a single enantiomer of chiral compounds, its use at industrial level is growing in importance for the production of pharmaceuticals and drug intermediates. To date, the majority of the bioprocesses carried out at industrial scale for pharmaceuticals use bacterial cell suspensions to perform enzymatic oxidations and reductions. It is expected that biotransformations will increase in importance due to the advances on genomics, proteomics and bioinformatics that will further the characterization of new systems and the engineering and use of new cell biocatalysts.