This chapter deals with plant metabolism studies conducted under regulatory guidelines to define the nature and quantity of the terminal residue of an agrochemical in crop plants and to develop residue definition for risk assessment and tolerance enforcement. Test plants selected from target crops are treated with the radiolabeled test substance at approximately 1 × maximum seasonal use rate as recommended in the use label of the agrochemical product. The RAC samples are harvested at appropriate growth stages, processed to powdery consistency and combusted to determine TRR levels. Samples containing significant TRR are extracted, and the metabolite profiles of the extracts are determined by radio-chromatographic methods. Metabolites are identified by co-chromatography with reference standards using two dissimilar analytical methods. Unknown metabolites are identified using various chromatographic, spectroscopic, and chemical methods. Unextracted bound residue is analyzed after treatment with surfactants, enzymes, acids, and/or bases. The metabolic pathways of the agrochemical in the test crop are postulated. The chapter also describes a plant metabolism case study and discusses the metabolic pathways of various groups of agrochemicals in plants.