The aerobic and anaerobic soil metabolism studies are conducted to measure the rate of transformation of agrochemical test substances in soil and to determine the nature and the rates of formation and decline of their transformation products. The findings from these studies help to regulate the use of the agrochemical, since its soil residue may be taken up by the crops, transported to groundwater, or carried by run-offs to surface water bodies, and affect human, aquatic organisms, plants, and other organisms. The essential soil parameters, experimental setup, incubation conditions, and sample analysis procedures to be followed under regulatory guidelines while conducting aerobic and anaerobic soil metabolism studies with agrochemicals are described in this chapter. The procedures for the determination of degradation rate and half-life (DT50) of the agrochemical using first order kinetics and PestDF method are described. The step-by-step conduct of a sample aerobic soil metabolism study is also presented. Examples of the metabolic pathways of various groups of agrochemicals in soil under aerobic conditions are given in the final section of the chapter.