Falling-weight deflectometers (FWDs) are used to collect important data that is used to characterize layer stiffness, and for planning maintenance and rehabilitation activities on highways and other major roads. Accurate data collection leads to a better prediction of maintenance and rehabilitation needs, which in turn leads to more effective planning and budgeting of these activities. Unfortunately, it is estimated that at this time only about half of the FWDs in use in the United States are regularly calibrated. This article will describe the primary sources of measurement error in FWD data collection and how the calibration protocol is used to effectively minimize these errors.