Based on the generally accepted fatigue mechanisms for brittle and ductile materials one would not expect single and polycrystalline silicon to be degraded by cyclic stresses. However, recent work has established that thin, micron-scale, films of single- and poly-crystalline silicon are susceptible to premature failure under cyclic loading conditions. Silicon films subjected to stress amplitudes of approximately one half of the single-cycle fracture strength exhibit lives ~109 cycles. However, the mechanism(s) of such failures have remained a mystery. In this presentation, we describe a mechanism for the fatigue of thin-film LPCVD polysilicon based on extensive stress-life fatigue testing and high-voltage transmission electron microscopy. It is proposed that the fatigue process is "surface dominated" and involves the mechanically-induced thickening of the native oxide film, followed by environmentally-assisted subcritical cracking of the thickened film.