To ensure the safe administration of vaccines to humans, vaccines (just like any new chemical entity) are evaluated in a series of nonclinical safety assessment studies that aim at identifying the potential toxicities associated with their administration. The nonclinical safety assessment of vaccines, however, is only part of a testing battery performed prior to human administration, which includes (1) the evaluation of the vaccine in efficacy and immunogenicity studies in animal models, (2) a quality control testing program, and (3) toxicology (nonclinical safety assessment) testing in relevant animal models. Although each of these evaluations plays a critical role in ensuring vaccine safety, the nonclinical safety assessment is the most relevant to the evaluation in human clinical trials, as it allows the identification of potential toxicities to be monitored in human trials, and in some cases, eliminates candidates that have unacceptable risks for human testing. This review summarizes the requirements for the nonclinical testing of vaccines and adjuvants needed in support of all phases of human clinical trials.