The transition from single-phase to two-phase thermal systems in future space vehicles demands a thorough understanding of flow boiling critical heat flux (CHF) in reduced gravity, including microgravity. This study is a comprehensive, consolidated investigation of the complex trends of flow boiling CHF in a rectangular channel in both microgravity and for different orientations in Earth gravity. It is shown that the Interfacial Lift-off Model provides good predictions of CHF data for both gravitational environments and both single-sided and double-sided heating. CHF mechanism in Earth gravity is shown to be highly sensitive to flow orientation at very low velocities, but is consistent with the wavy vapor layer depiction of the Interfacial Lift-off Model at high velocities. The model predicts a stable vapor-liquid interface for downflow with a downward-facing heated wall at lower velocities, and wavy interface with a critical wavelength that decreases with increasing velocity at higher velocities. Predicted CHF values for microgravity fall about midway between the maxima and minima for Earth gravity. Overall, predicted values of CHF and key interfacial parameters for all orientations in Earth gravity and for microgravity converge above similar to 1.5 m/s, which points to a velocity threshold above which inertia begins to effectively negate gravity effects. (C) 2015 Elsevier Ltd. All rights reserved.