Experiments were performed to explore the trigger mechanism for critical heat flux (CHF) on 10-, 30- and 110-mm long heaters which were mounted along one wall of a rectangular channel. The channel was oriented vertically, horizontally, and at an intermediate 45 degrees angle with liquid FC-72 flowing above the heater surface. Using photomicrographic methods, still photographs and high speed video images were obtained normal to the heater surface through a transparent cover. The photographs revealed the formation of a wavy vapor layer on the heater prior to CHF with surface wetting occurring al the wave troughs. The vapor waves formed on the upstream portion of the heater and had wavelengths characteristic of the Kelvin-Helmholtz instability. The distance between wetting fronts increased in the stream-wise direction because of wave stretching and merging of adjacent waves. Eventually, midway down the long heater, the distance between the wetting fronts reached a constant asymptotic value. The overall impact of the stream-wise increase of wavelength was a reduction in the number of wetting fronts available for liquid replenishment of the heater surface causing a decrease in CHF with increasing heater length. Critical heat flux was triggered by lifting of the most upstream wetting front spurring a cessation of wetting front formation and a catastrophic cascading of wetting front lift off along the entire heater.