Craze formation determines the effective strength of many structural polymers and glassy polymer adhesives. We use molecular dynamics simulations to study the evolution of craze formation in glassy polymers as the number of monomers per chain N increases. When N is less than the entanglement length N-e, the polymer cavitates and then fails along a simple fracture plane. As N becomes greater than N-e, the mode of failure changes, and the entire volume deforms into a craze. As in experiment, crazing occurs at a constant plateau stress S and expands the volume of the polymer by a fixed extension ratio lambda.