The three primary study objectives were: (1) to characterize the performance (pecision and accuracy) of portable XRF instruments under field conditions; (2) to evaluate the effect on XRF performance of interference from the material (the substrate) underlying the paint; and (3) to characterize the relationship between test kit results and the actual lead level in the paint (operating characteristic curves). The three secondary study objectives were: (4) to understand XRF behavior in the field through the investigation of XRF measurements that were very different than their corresponding lab result; (5) to evaluate field quality assurance and control methods; and (6) to investigate the variability of lead levels in the paint within the study sampling locations.