The strength of glass is known to be very variable and also to be affected by the condition of the surface. By improving a technique of decoration with sodium, due originally to Andrade, elaborate crack systems have been revealed on the surface of drawn glasses. These crack systems are correlated with, and may control, the mechanical strength of glass. They also appear to indicate the existence of a tension layer on the surface of glass. The cracks seem to be 100 to 200 angstrom wide and are probably at least 1000 angstrom deep. Cracks can be initiated by local abrasion, but this is probably not the only cause. A technique for the examination of thin glass films by transmission electron microscopy has also been developed and has been used to observe the mechanism of devitrification in various glasses. The initiation of cracks in fine crystallites has been watched and such cracks have been seen to propagate from the crystalline to the glassy regions in silica. It is, therefore, possible that fine-scale devitrification on the surface of glass during drawing might provide an alternative mechanism to abrasion for the origin of the surface cracks.