Photoelasticity has, until the last few years, been, in the main, restricted to two-dimensional examination because of the limitations of model-making materials. Requirements for three-dimensional model-making plastics are discussed in detail, and the faults of the few materials used to date listed. The introduction of a cross-linked polyester resin, readily available, into this field permits further investigations to be conducted. The chemical and physical properties of this new resin are considered, and a theory explaining the frozen effect propounded. The resin is castable in the user's laboratory, and provided adequate care is taken, castings can be obtained to the near finished size. Mottle, which, it is thought, is developed during the setting period of the resin, and edge effects which appear during the freezing cycle, are the main limitations of this new plastic. Methods of overcoming both these defects have been devised and the techniques adopted are described in detail. The results indicate that the figure of merit (a criterion which may be adopted as an indication of the sensitivity of the material) of this particular resin is only slightly lower than that of the latest American material, Fosterite.