The effects of orientation on flow boiling critical heat flux (CHF) were investigated using high-speed video and microphotographic techniques. Interfacial features were measured just prior to CHF and statistically analyzed. A dominant wavy vapor layer regime was observed for all relatively high-velocities and most orientations, while several other regimes were encountered at low velocities, in downflow and/or downward-facing heated wall orientations. The interfacial lift-off model was modified and used to predict the orientation effects on CHF for the dominant wavy vapor layer regime. The photographic study revealed a fairly continuous wavy vapor layer travelling along the heated wall while permitting liquid contact only in wetting fronts, located in the troughs of the interfacial waves. The waves, which were generated at an upstream location, had a tendency to preserve a curvature ratio as they propagated along the heated wall. CHF commenced when wetting fronts near the outlet were lifted off the wall. This occurred when the momentum of vapor normal to the wall exceeded the pressure force associated with interfacial curvature. The interfacial lift-off model is shown to be very effective at capturing the overall dependence of CHF on orientation.