In this paper, we discuss the effect of water on the strength and static fatigue of silica glass. When a crack is formed in silica glass, the surrounding environment rushes into the crack; water then diffuses from the environment into the newly formed fracture surfaces to generate a zone of swelling around the crack tip. Because the swollen material is constrained from expanding by the surrounding glass, a zone of compressive stress is generated at the fracture surface around the crack tip. The results are similar to those found for transformation toughened zirconium oxide, with the exception that the transformation zone in silica glass grows with time, so that the effect gets progressively stronger. Using diffusion data from the literature, we show that the diffusion of water into silica glass can explain several significant experimental observations: the reported strengthening of silica glass by soaking in water at 88 degrees C; an increase in the slope of dynamic fatigue curve by prior exposure to water at 88 degrees C; the observation of a static fatigue limit in silica glass at very low values of the applied stress-intensity factor; and the observation of crack face displacements caused by water penetration into the glass at the crack tip.