Vaccines are one of the most cost-effective public health interventions in existence. Because they directly prevent substantial morbidity and mortality, vaccines can indirectly expand opportunities for health care by sparing health care resources that would otherwise be needed to care for individuals with preventable infectious diseases. Realizing the direct and indirect benefits of vaccines requires an investment of resources to provide the core functions of immunization programs. And this is the challenge—in low-resource communities, how is such an investment made and sustained?