Entrance effects can persist for great lengths, L/D = 300, and long times, in developing two phase flows. Wall shear stresses at moderate and low velocities contribute only slightiy to the pressure drop in slug flows. Bubblerise velocities in slug flow are quite sensitive to the velocity profile in the water ahead of the bubbles. The effect of wall shear stress manifests itself pnimarily through the velocity profiles. Pressure drop, density, slug length, bubble length and fluctuation pressure drops, and frequencies can be calculated for fully developed slug flow with gocd accuracy.