Vaccines are responsible for the control of many infectious diseases that were once common in the United States, including polio, measles, diphtheria, pertussis (whooping cough), rubella (German measles), mumps, tetanus, andHaemophilus influenzaetype b. National efforts to generate collaboration between federal, state, and local governments and public and private health care providers have resulted in record high levels of vaccination coverage in the United States. The high rate of US vaccinations is paralleled by growing concerns about the safety of their delivery. The variety of substances used in vaccines sometimes causes the development of cutaneous reactions in susceptible adults and children. This article will review adverse cutaneous events consistent with hypersensitivity reactions to the following ingredients in vaccines: aluminum, thimerosal, 2-phenoxyethanol, formaldehyde, and neomycin.