Generic drug products are becoming increasingly important to health care. Nearly 80% of prescriptions filled in the United States are for generic drugs. At the US Food and Drug Administration (FDA), the Office of Generic Drugs (OGD) is responsible for the approval of generic drug products. The OGD assesses the safety, efficacy, and quality of generic drug products by making a finding of pharmaceutical equivalence and “bioequivalence.” A key principle is that generic drug products that reach health care providers and patients must be scientifically verified as appropriately substitutable for the brand-name products that are listed by the FDA as reference products.