Bicycling is a popular recreational activity and a principal mode of transportation for children in the United States, yet about 300 children die and 430,000 are injured annually. Wearing a bicycle helmet is an important countermeasure, since it reduces the risk of serious brain injury by up to 85%. The Centers for Disease Control and Prevention (CDC) have funded state health departments to conduct bicycle helmet programs, and their effectiveness has been evaluated by monitoring community bicycle helmet use. Although it would appear that measuring bicycle helmet use is easy, it is actually neither simple nor straightforward. The authors describe what they have learned about assessing helmet use and what methods have been most useful. They also detail several key practical decisions that define the current CDC position regarding helmet use assessment. Although important enough in their own right, the lessons learned in the CDC's bicycle helmet evaluation may serve as a model for evaluating other injury prevention and public health programs.