This book covers a fruitful literature review of the anaerobic process. It includes a description of the different kinds of anaerobic reactors, emphasizing on UASB reactor. It takes into account the granules development, the kinetics models, and a review of the existing models describing the UASB reactor behavior. The main purpose of this work is the development of a model able to predict the substrate degradation, the biomass growth, and the location of the granule size along the height of the reactor. Based on knowledge about transport phenomena, a model was developed in order to describe the processes occurring in the UASB reactor. It was tested using data from the literature and software (Matlab, Comsol) was used to run the model.