Control of vaccine-preventable diseases depends on maintaining high levels of immunization coverage. Immunization coverage among preschool children remains suboptimal in some areas and sociodemographic subgroups, as well as for more recently introduced vaccines, leaving susceptible young children vulnerable to complications from vaccine-preventable diseases. This paper reviews approaches historically used to measure immunization coverage among preschool children in the United States. The strengths and weaknesses of various approaches to measuring immunization coverage among preschool children are explored, with emphasis on the current means to measure national immunization coverage—the National Immunization Survey. Methods for measuring immunization coverage among preschool children at local and state levels are also evaluated. Future opportunities and challenges for measuring immunization coverage at the local, state, and national levels are explored.