This chapter introduces microbial kinetics as the framework for quantifying the rates of microbial growth and substrate utilization in environmental systems. It begins with the bacterial growth curve and its phases, emphasizing exponential growth as the basis for calculating generation time and growth rate constants. The Monod equation is presented as the cornerstone model, linking substrate concentration to specific growth rate through key parameters such as μmax and Ks. Concepts of true and net yield are discussed, showing how biomass production relates to substrate consumption and decay processes. By integrating stoichiometric and kinetic parameters, the chapter equips readers with tools to design, analyze, and optimize biological treatment reactors for wastewater engineering.