In some situations where it is not ethical or feasible to conduct efficacy studies in humans, animal studies may be done to determine the efficacy of drugs and biological products for serious or life-threatening conditions. This is particularly true in the case of the development of countermeasures to chemical, biological, radiological, or nuclear threat agents. The choice of the animal model(s) and the design and implementation of the studies are challenging since the efficacy results of the animal studies need to be predictive of the response expected to occur in humans. In an attempt to provide guidance on conducting such studies the Food and Drug Administration (FDA) has a draft guidance document entitled “Animal Models-Essential Elements to Address Efficacy Under the Animal Rule.” This guidance commonly known as the “Animal Rule” guidance focuses on the identification of the critical characteristics (essential data elements) of an animal model to be addressed when developing drug or biological products for approval or licensure respectively under the Animal Rule [21 Code of Federal Regulations (CFR) 314.600 for drugs; 21 CFR 601.90 for biological products]. While animal studies are relied upon to determine the efficacy of drugs or biological products under the “Animal Rule,” the safety of the drug or biological product is determined in humans. It is important to note that the “Animal Rule” does not apply to products that could be approved based on efficacy standards described elsewhere in FDA's regulations. The “Animal Rule” guidance is found at http://www.fda.gov/downloads/Drugs/GuidanceComplianceRegulatoryInformation/Guidances/ucm078923.pdf