School immunization laws have had a remarkable impact on vaccine-preventable diseases in the United States, particularly in school-aged populations. Enforcement of laws through the exclusion of unvaccinated children from school is a critical factor in assuring success. All laws have exemptions for medical contraindications, 47 states have exemptions for persons with strong religious beliefs against vaccination and 15 states have exemptions for persons philosophically opposed to vaccination. Fewer than 1% of students have any type of exemption in most states. School laws harness the resources of other programs such as education to the immunization effort. They establish a safety net to assure high levels of coverage each and every year. But they cannot replace efforts to assure age appropriate immunization in the first two years of life.