Theoretical equations governing slip effects in forced circulation of boiling water are derived. The equations indicate that steam slip is dependent upon channel geometry, inlet water velocity, and rate of heat addition. A simplified momentum model is postulated which leads to equal friction and head losses of two phases. The model gives good agreement with available experimental results in horizontal and vertical test sections with and without heat addition at pressures from 12 to 2000 psia. Discussion of the model in terms of nonquasi steady-state unbalances of friction and head losses of the two phases explains experimental deviations from the predictions and the previously noted effects of water inlet velocity. It also gives trends for the effects of channel geometry and rate of heat addition. Application of the simplified model to calculating two-phase pressure drops is included.