In this study, non-intrusive pressure drop, liquid base film thickness distribution, and wave behavior measurements have been obtained for 206 horizontal annular two-phase (air-water) flow conditions in 8.8, 15.1, and 26.3 mm ID tubes. Wall shear was correlated to within 8% by a friction factor involving flow quality and gas Reynolds number. This correlation was found to perform better than those available in the literature, including film roughness correlations, two-phase multiplier methods, and pure data fits. Among published relations, the Muller-Steinhagen and Heck correlation was found to be the most accurate, while the Lockhart-Martinelli correlation can be modified to provide reasonable results. The gas friction velocity is found to be similar to the disturbance wave velocity, which suggests that waves are important sources of shear. (C) 2008 Published by Elsevier Ltd.