The agrochemical released in the environment undergoes partial or complete structural changes after being acted upon by the abiotic and biotic factors. Degradation is the term applied to various structural changes in the agrochemical molecule caused by the abiotic physicochemical factors. The structural changes caused by biotic factors such as enzymes in living organisms are termed metabolism. Agrochemical degradation and metabolism are multistep transformation processes producing serial as well as parallel pathways. Metabolism usually occurs in four different phases. In Phase I, the parent molecule and its phase I product undergo transformation through oxidation, reduction, hydrolysis, photolysis, rearrangement, substitution, and other similar reactions. In Phase II, the parent molecule, or its phase I transformation product conjugates with an endogenous molecular moiety of the host organism through covalent bonding. The Phase III metabolism involves conjugation of parent molecule, or its phase I or phase II transformation product with natural structural components of the host organism or the soil matrix producing bound residues that are normally insoluble in water and common organic solvents. Phase IV metabolism occurs when the radiolabeled atom (usually 14C) from the agrochemical is incorporated into natural macromolecules of the host organism. Processes involved in various phases of metabolism are discussed in this chapter.