As the need for highly selective chemistry grows, the power of enzyme and whole-cell mediated catalysis is being increasingly recognised. Indeed, biotransformations are now finding application throughout the pharmaceutical industry and this growth seems set to continue. However the majority of reactions operating commercially are hydrolytic resolutions and there is a need now to introduce more complex biotransformations such as those for carbon-carbon bond formation and in particular redox conversions. Frequently implementation of a new biotransformation is limited by low productivities and a lack of design information. In this lecture I will outline process modelling techniques we have been developing at UCL to assist in identification of the process constraints. For redox conversions a common constraint is the lack of enzyme stability under operational conditions. Enzyme immobilisation by covalent attachment is one practical method of approaching this problem. Results of enzyme immobilisation on Eupergit C (Rohm, Darmstadt, D) for ketone reduction and Baeyer-Villiger oxidation by cyclohexanone monooxygenase will be presented to indicate some of the limitations and the need for more data on support materials for biocatalyst immobilisation.