Early stage process analysis to maximize economic feasibility and minimize environmental burden is critically important for process flow sheet selection and optimization. This is equally true for bioprocesses. For many systems, including bioprocess systems, the desired material and energy balance data are not available at an early stage of design. The CeBER Bioprocess Modeller (Centre for Bioprocess Engineering Research at the University of Cape Town, Department of Chemical Engineering) has been developed to provide estimates of these material and energy balance data, as well as calculate equipment volumes and utility needs.The model allows for aerobic or anaerobic, intra- or extracellular product formation or biomass growth in a continuous or batch process for various bioproducts produced from a selection of microorganisms using a range of raw materials. Using simple inputs, and default values drawn, the model is suitable for engineers and scientists alike.The model also incorporates the option to select for sterilization and to specify the downstream processing train. In the bioreactor, the model takes into account aeration, agitation, reaction, biomass maintenance, yield, post microbial growth cooling and growth rate calculations, amongst others. This paper presents the model framework for the microbial growth and product formation stages.