Photographic studies of vertical pool boiling for near-saturated conditions were conducted in order to determine the critical heat flux (CHF) trigger mechanism. The studies revealed that, for heat fluxes near CHF, vertical pool boiling exhibits vapor behavior similar to that observed in flow boiling. At fluxes slightly below CHF, the Kelvin-Helmholtz instability creates a wavy layer at the liquid-vapor interface. The heater surface is wetted, and thus cooled, only by the wave troughs, referred to as wetting fronts, which repeatedly sweep across the surface. Prior to CHF, the curvature of the wetting front creates a pressure force which tends to preserve interfacial contact with the wall. Critical heat flux occurs when the pressure force due to interfacial curvature is overcome by the momentum of vapor at the site of the first wetting front, causing the interface to lift away from the wall. Once the first wetting front lifts off from the surface, the remaining wetting fronts separate from the surface in succession. It is shown that the interfacial lift-off criterion facilitates accurate theoretical modeling of CHF in vertical pool boiling, and produces an equation which is identical in form, but not mechanism, to traditional horizontal CHF models.