The regulatory aspects of generic drug substitution and the scientific concepts that serve as the basis for generic drug approval are discussed, with emphasis on the source of therapeutic equivalence information compiled by the Food and Drug Administration in Approved Drug Products with Therapeutic Equivalence Evaluations. The Food and Drug Administration's determination of bioequivalence for immediate-release and extended-release dosage forms is summarized, with a discussion of the underlying assumptions and current issues regarding bioequivalence testing. Medical practitioners must comply with the regulations stated in each state's Pharmacy Practice Act when allowing generic substitution and should ensure that the substituted product is therapeutically equivalent to the prescribed product.