Velocity distributions, friction losses, and heat-transfer characteristics were studied analytically and experimentally for fully developed turbulent flow in tubes with twisted tape swirl generators. Data were obtained for pitch-to-diameter ratios from 3.62 to 22.0 with both air and water under isothermal and forced convection heating conditions. The following principal conclusions result: (a) The velocity field is helicoidal and corresponds to a forced vortex in the core superposed on an essentially uniform axial flow. (b) Friction losses may be predicted from the combined effects of the axial and tangential boundary-layer flows coupled with an additional “vortex-mixing” effect. (c) Approximate heat-transfer correlations can be predicted from a Colburn-type analysis. These may be improved by considering a boundary layer and vortex mixing model.