The medical dictionary defines immunization as the “protection of susceptible individuals from communicable diseases by the administration of a living modified agent, a suspension of killed organisms, or an inactivated toxin.” This elegant description can be expanded to include twenty-first century approaches to immunization that include recombinant technology, reassortment virus techniques, live vectors, DNA vaccines, and the expansion of the field to encompass noncommunicable diseases such as Alzheimer's disease, autoimmunity, and tumor immunogenetics. Integral to the success of immunization is our knowledge of the immune system's memory of antigens, yet our understanding of this fundamental feature remains limited. On a global scale, communicable diseases remain the number-one cause of morbidity and mortality; hence Jenner's pioneering work with its birth in 1796 still has a challenging and exciting future. (J Allergy Clin Immunol 2003;111:S754-65.)