Publisher Summary This chapter presents the pharmacodynamics of drug-receptor interactions. Because of the low concentration of drugs and receptors in the bloodstream and other biological fluids, the law of the mass action alone cannot account for the ability of small doses of structurally specific drugs to elicit a total response. The biological activity of a drug is related to its affinity toward its specific receptor. This stability is measured by κd, the dissociation constant for the drug-receptor complex at equilibrium. The drug-receptor complex formation is considered entropically unfavorable because of the loss in conformational degrees of freedom for both the protein and the ligand. Therefore, highly favorable enthalpic contacts (interactions) among the receptor and the drug compensate for the entropic loss. The interactions involved in the drug-receptor complex are the same forces experienced by all interacting organic molecules. Weak interactions are possible when molecular surfaces are close and complementary. The selectivity in drug-receptor interactions can result several different types of molecular interactions.