Photoelastic methods of stress analysis have so far been limited to the study of problems of elasticity. The usefulness of such methods has been extended to cover inelastic behaviour by showing that the relative retardation due to loading is determined by the stress and strain components independently of the history of loading. A relation expressing relative retardation as a linear function of stress and strain differences has been derived. This fundamental relation has been verified experimentally for a wide variety of model materials. A fundamental procedure for photoplastic investigations is proposed. This consists of taking two fringe photographs of the same model, one under load, the second immediately after removal of the load. The difference between the fringe counts at corresponding points gives the stress difference which in elastic problems is derived from a single photograph.