Vaccines are considered one of the greatest public health achievements of the last century for their role in eradicating smallpox and controlling polio, measles, rubella, and other infectious diseases in the United States. Despite their effectiveness in preventing and eradicating disease, substantial gaps in vaccine uptake persist. Vaccination rates for young children are high; however, vaccination rates remain well below established Healthy People 2020 targets for many vaccines recommended for adolescents, adults, and pregnant women. In the United States, vaccine guidelines are set by the Centers for Disease Control and Prevention's Advisory Committee for Immunization Practices (ACIP). The number of routine immunizations recommended for children and adolescents, adults, and pregnant women has expanded considerably over the past 10 years. As the number of recommended immunizations has expanded across the population, so too have concerns about the safety of vaccines. Perhaps the most highly publicized safety concern of the last two decades was the proposed link between autism and the measles, mumps, and rubella (MMR) vaccine. Although multiple large studies have confirmed the lack of association between MMR and autism, parental worries about the safety of vaccines persist. Other parental concerns about childhood vaccines include potential links to multiple sclerosis, sudden infant death syndrome, asthma, and diabetes. Thus, vaccine safety is high on the Nation's public health agenda. This report, which represents the results of a comprehensive and systematic review of scientific evidence, describes potential associations between vaccines and adverse events (AEs).