The role of internal stresses in the formation of voids during cure of neat epoxy resin and fibre reinforced epoxy has been identified. The combined evolution of the modulus and of the volume shrinkage often leads to voiding when the resin is constrained during cure. To analyse this phenomenon, the shrinkage of an epoxy resin was studied in a constrained volume. Void initiation was observed at two different stages during isothermal cure, before gelation and around or after gelation. A corresponding critical internal stress at void initiation was calculated by viscoelastic analysis. This stress criterion is used to construct a process window for production of void-free composites.