The mission of the US Food and Drug Administration (FDA) is to protect public health by ensuring the safety, efficacy, and quality of drugs. In drug development, animal studies play a key role in assessing the safety (toxicology), pharmacokinetics (PK), and proof-of-concept efficacy of a new product. When clinical studies are neither ethical nor feasible to conduct, the Animal Rule (67 FR 37988, 2002) introduces the potential for drug (e.g., countermeasure) approval based on efficacy studies in animals, and appropriate human safety and pharmacokinetic information. The Center for Drug Evaluation and Research is responsible for the review of drug and therapeutic biologic applications. The use of the word “drugs” in this paper will henceforth refer to both drugs and therapeutic biologics (e.g., monoclonal antibodies and small protein molecules). Information about vaccines and other biologics, such as antisera and blood products, should be obtained from the Center for Biologic Evaluation and Research.