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A new fracture test specimen has been developed to study crack extension and arrest in nuclear reactor vessel steels subjected to stress-intensity factor and toughness gradients similar to those in postulated pressurized thermal shock situations. A summary of the results of all the tests performed is presented to illustrate the range of crack arrest and crack reinitiation conditions observed. One test of this specimen with the corresponding stress analysis is described in detail. During this test the crack initiated, extended, arrested, reinitiated, extended again, and reached a final arrest. Comparison of detailed dynamic elastic-plastic finite-element analyses and dynamic strain and displacement measurements of the crack extension, arrest, and reinitiation events, combined with topographic analysis of the fracture surfaces, has led to a new understanding of the crack extension and arrest process. The results of the tests demonstrate crack arrest in a rising stress-intensity field at near-upper-shelf temperature conditions and show that the toughness required for arrest is lower than would be predicted by the analysis procedures usually employed for pressurized thermal shock evaluations.