The results of an extensive experimental investigation of the thin water films of thickness less than 0.38 mm (0.015 in.) on the tire-pavement friction are presented. Both laboratory and full-scale field tests involving new measuring equipment were conducted, and a new device was developed for laboratory measurement of rubber-surface friction. Also, a new apparatus was built to measure thickness of thin water films formed on pavement surfaces. The results from the laboratory and field tests indicate that the tire-pavement friction can decrease significantly at very thin water films from 0.025 to 0.230 mm (0.001 to 0.009 in.). A quantitative measure of the effect of water-film thickness on tire-pavement friction is proposed. The effects of tire, vehicle speed, and pavement texture on pavement wetness sensitivity are also investigated.