A list of the microorganisms that can be cultivated for obtaining chemical products or biomass is reported in the introduction of this chapter. Fermentation processes and microorganism kinetic growth are then described. The historical evolution of the kinetic models, for the interpretation of the fermentation processes, is successively reported, considering both unstructured and structured models with their respective variants of segregated and unsegregated models. Starting from the unstructured Monod kinetic model and related modifications the compartmental structured models are then considered, in particular metabolic and cybernetic model. These models have been applied to the production of baker yeast from Saccharomyces cerevisiae, as a useful example of microorganism kinetic growth. It has also been considered the production of bioethanol, always employing the same microorganism but in anaerobic conditions. At last, a brief review of the nonmechanistic models such as neural network mathematical models and fuzzy models is reported at the end of the chapter. Different examples and solved exercises, related to both unstructured and structured models, described in the chapter are reported along the text.