The use of stress-optical measurements to characterize oriented PET fibers is reviewed. The implications of a molecular network for the development of orientation in spun fibers, cold drawn and hot drawn fibers are also considered. Early research made use of the Kuhn and Grun theory of photoelasticity, and their model has been shown by many workers to be a useful starting point for understanding the development of molecular orientation in PET provided that it is suitably modified for high chain extension at high draw ratios. The principal thrust of recent research at Leeds has been to explore the applications of more precise molecular modelling using the Monte Carlo rotational isomeric state model developed by Stepto and Taylor, and applying it to the development of optical birefringence with strain, as well as the shrinkage stress. It has been shown that the Kuhn and Grun theory, where the actual network is replaced by a model network of freely jointed random links, cannot provide a consistent description of the shrinkage stress and the optical birefringence.