The behavior of tough, tough, crystalline thermoplastics in notched impact tests leads to the definition of crack initiation resistance and propagation resistance as two distinct properties, G(c) and G(D). It is shown here that a single criterion-adiabatic thermal failure of a crack-tip cohesive zone-can be applied to predict both. Dynamic fracture resistance G,emerges as a geometry independent, though crack speed and temperature dependent, material property, whose minimum value G(D,min) depends only on temperature and bulk physical properties. G(D,min) can be measured using a scruple pressurized-tube test. Crack initiation resistance G(c), however, is inherently influenced by geometry and impact speed, although its lower bound is also G(D,min). Craze extension and failure of a notched impact specimen, and hence G(c), can be predicted for a specific temperature, given bulk thermal property data and a dynamic stress/strain curve measured by impact bending of an unnotched beam. For materials that comply with the model, sharp-notched Charpy type impact tests will not arrive at a unique G(c) value, while Izod type tests, for which a revised compliance calibration is presented, may fail to establish any G(c) value at all.