Despite the impact vaccination has had on the control and prevention of many infectious diseases, some parents choose not to vaccinate their children. Although there is no federal law requiring vaccination of children in the United States, all states require evidence of vaccination against at least some diseases as a condition of school entry. Which vaccines are required; how many doses are required; whether entry requirements apply to child care, kindergarten, or middle school; and whether exemptions from vaccine requirements will be allowed all differ by state. All but two states allow some kind of personal belief exemption from school vaccination requirements. This article reviews the history of school vaccination requirements and exemptions, the legal status of state vaccination laws and exemptions, the impact of school vaccination requirements and personal belief exemptions on vaccination rates and disease incidence, and strategies for maintaining adequate vaccination rates in states that allow personal belief exemptions.