The work of fracture of a number of epoxide resins has been measured at room temperature, 77K, and 4.2K, using a three-point-bending method with deeply notched specimens. Various types of yield behavior were noted, and the results and observations have been used to classify resins as brittle or semi-flexible at room temperature and to record the presence of plasticity even at lower temperatures. For semi-flexible resins at room temperature, the geometry dependent elastic/plastic contribution to the work of fracture is discussed. This work indicates how the work of fracture and the glass transition temperatures of the resin may be used to guide developments of resins for low temperature applications and possibly to derive a numerical term as a "Cracking Index." These results were used to explain the observed thermal shock resistance of these materials. A scanning electron microscope (SEM) and optical microscopy were used to investigate the fracture surfaces associated with the different types of yield and fracture behavior. The fracture surface microstructure has been used to explain the observed mechanical behavior, including the existence of plastic deformation in some resins at 77K. The results are of help in understanding the process of crack initiation, growth, and the crack tip deformation processes in epoxide resins.