The kinetics of microbially catalyzed reactions are of special interest because of the control the reactions exert of the redox state of laboratory experiments and the natural environment. A general description of microbial kinetics must address the requirement of thermodynamic consistency, so the kinetic laws apply equally well far from chemical equilibrium, and close to it. This chapter shows how to formulate thermodynamically consistent rate laws for microbial respiration and fermentation, the process of incorporating such laws into multicomponent chemical reaction models, and a fully worked example demonstrating how such models behave.