Americans serving with the US Armed Forces need protection from the dangerous infections that they can contract during training, based on occupation, during overseas deployment, or because of underlying health status. For over 230 years, the military health-care system has immunized troops to protect them personally and to help them accomplish their missions. Military researchers have invented, developed, and improved vaccines and immunization delivery methods against more than 20 diseases. This article consolidates content from several previous historical reviews, adds additional sources, and cites primary literature regarding military contributions and accomplishments. Discussion emphasizes smallpox, typhoid fever, tetanus, influenza, meningococcal disease, adenovirus, yellow fever, pneumococcal disease, and anthrax. Delivery issues include documentation, simultaneous immunization, seroscreening, safety surveillance, jet injection, and cold-chain management. Immunization policies for each major US conflict are described. Military immunization programs need to be individualized on the basis of personal contraindications and prior immunity. The proper conduct of military immunization programs respects the need for detailed education of military personnel, maximizes quality in immunization delivery, and supports quality clinical care to prevent and treat adverse events after immunization. Military immunization programs maintain the health of soldiers, marines, sailors, airmen, and coast guardsmen, the resources most critical to military success.