Combination vaccines have been in use for >50 years. Historical problems with vaccines, including intussusception after rotavirus vaccine, carrier suppression with tetanus toxoid conjugate vaccines, and decreased immunogenicity of some Haemophilus influenzae type b conjugate vaccines when mixed with acellular pertussis-diphtheria-tetanus, have contributed to some misperceptions about current vaccines. There is no evidence that adding additional vaccines through combination products increases the burden on the immune system, which has the capability of responding to many millions of antigens. Combining antigens usually does not increase adverse effects-in fact, it can lead to an overall reduction in adverse events. Combination products simplify immunization and allow for the introduction of new vaccines without requiring the vaccinee to make additional visits to his or her health care provider. Licensed combination vaccines undergo extensive testing before approval by the United States Food and Drug Administration to assure that the new products are safe and effective.