This study compared States with high seat belt use rates and States with low seat belt use rates using statistical analyses and conducted case studies of 10 high seat belt use States. Primary enforcement seat belt use laws clearly help increase seat belt use, though three case study States with secondary laws have achieved high seat belt use. High-visibility seat belt law enforcement is critical. The statistical analyses suggest that the most important difference between the high and low seat belt use States is enforcement, not demographics or funds spent on media. A few geographic, demographic, and cultural factors are associated with lower seat belt use, but none is a barrier to high seat belt use, as demonstrated by examples from the case study States. The key factors in all high seat belt use case study States were high-visibility seat belt law enforcement, excellent relations with law enforcement command and officers statewide, effective seat belt law enforcement publicity, high priority for increasing seat belt use, effective planning and implementation of seat belt use programs based on solid data and research, and effective Highway Safety Office and seat belt program management. Recommendations for States wishing to increase seat belt use are included.