Stoichiometry deals with chemical reactions and the numerical relationships between the reactants and the products. Chemical reactions are classified based on their reaction kinetics. Reaction kinetics is the study of the rates of chemical reactions at which reactants are consumed, or products are formed in a stoichiometric reaction. The following objectives will be reviewed in this chapter: stoichiometric homogeneous and heterogeneous reactions; reaction rates: zero-order, first-order, second-order, and saturation-type and enzymatic reactions; effect of temperature on reaction rate; reaction order data analysis and design; nitrification reactions, equations and solutions, and graphical representation; parallel irreversible reactions; derivation and application of type II of second-order reactions.