Publisher Summary This chapter discusses methods of analysis of drugs and drug metabolites. Pharmacokinetics (PK) requires the determination of a concentration of a drug, its metabolite, or an endogenous targeted substance in physiological fluids or tissues with respect to time. Chromatography refers to the separation of materials using their relative solubility and absorption differences in two immiscible phases: one stationary and the other mobile. When a component is more soluble in the mobile phase than in the film on the particle, it will elute from the column and will be detected with respect to a characteristic chemical property, such as ultraviolet (UV) absorption. An antigen, such as a drug, can be labeled with a fluorescent tag, and the fluorescent emission of polarized light is measured in a competitive antibody-binding assay. Immunoaffinity-based assays are routinely developed for new biologicals and products of the biotechnology industry as a part of their characterization as new agents. Selection of a suitable assay method for these compounds began with the consideration of the chemical characteristics of the drug and the determination of the likely range of blood and tissue concentration required for pharmacological effect.