Routine immunization is the foundation through which countries provide access to lifesaving vaccines and control and eradicate vaccine-preventable diseases. It is the process of timely vaccination on a regular basis with vaccines considered important for a given country to reduce morbidity and mortality. This process is enabled by a country’s health system and maintained through a set of management subsystems needed to continuously supply the full complement of scheduled vaccines, monitor their safety, control population coverage, and measure their epidemiological impact. Essential components of immunization systems align with fundamental health systems components and function within the context of broader health and social welfare systems.