Small angle X-ray scattering has been used to study the mechanisms of craze induced failure in polystyrene. The failure processes have been examined for both constant deformation and fatigue situations. It was possible to separate the craze volume in the beam into a loaded portion and an unloaded portion, thereby showing that the total volume of craze matter continued to grow even after most of the crazes had been unloaded by crack propagation. In fatigue, as a craze is unloaded, the fibrils initially contract and as the stress across them becomes compressive, they buckle. The main reduction in life of a sample by fatigue occurs during the contraction and re-extension of the fibrils, not during the buckling process. An examination of long-term changes of SAXS patterns during fatigue has shown that fibrils coalesce during the fatigue process.