It is estimated that 4 million children die each year of vaccine-preventable diseases and that another 4 million are permanently disabled. Although vaccination is the most cost-effective health technology, there is a gap between what we know and what we apply. We seem to be succeeding more in attaining new knowledge than in applying what we know. It takes more than a string of discoveries to provide the benefits of science to the people. Appropriate application of the vaccines presently available against diseases such as measles, poliomyelitis, neonatal tetanus, and pertussis could result in a significant reduction in mortality and morbidity among infants and children attributable to these diseases; continued development and deployment of newer vaccines against some dominant components of the diarrhea-dysentery-pneumonia complex of illnesses could result in further reduction. Improved management practices; more heat-stable, less-expensive, and safer vaccines; and vaccines that require fewer doses are critical for success in future immunization programs.