This thesis presents a combination of numerical and experimental studies performed to assess the influence of the warm prestress effect on the cleavage fracture toughness of two ferritic pressure vessel steels. The aims of the research are to gain a detailed knowledge of the materials low temperature response under uniaxial and fracture conditions; to examine, using the finite element method, crack tip stress fields during warm prestress LUCF load cycles; and provide a clear and consistent method of classifying the warm prestress effect. An experimental programme investigated the room temperature and low temperature response of two candidate steels, A533B and BS1501. These steels were tested uniaxially under monotonic and cyclic conditions, and in the cracked condition in the as-received and warm prestressed conditions. Application of a three parameter statistical model to the experimental data showed that the distribution of data in the as received and warm prestressed conditions can be described accurately. The shift in the cleavage toughness distribution following warm prestressing was predicted by combining the statistical model with a validated analytical model of the warm prestress effect. Repeated proof loading was shown to increase cleavage toughness in A533B steel, providing the loading was load controlled. There were negligible effects of repeated proof loading on BS1501 steel. Some further enhancement of cleavage fracture toughness was observed when sub critical crack extension was introduced following warm prestressing, although the results were highly scattered. The finite element method was employed to simulate experimental fracture events. It was found from these simulations that fracture occurs following warm prestressing, when the reloaded crack tip stress distribution matches the as-received fracture crack tip stress distribution. The stress matching was observed to occur well into the elastic stress field ahead of the crack tip. This fracture criterion was employed to provide predictions of cleavage toughness following varying applied preload levels. The results were compared to experimental data sets and various analytical models. The Chell model of the warm prestress effect was observed to provide the best agreement with the finite element predictions. Crack tip blunting during the preload steps was found to have no influence on the predictions of cleavage fracture toughness. Differences in hardening response of the material was also shown to have little influence of the predictions of cleavage toughness. Simulations incorporating sub critical crack extension prior to reloading to fracture demonstrated that cleavage 'toughness can be enhanced further by limited crack extension. Large increments of crack growth were shown to reduce the warm prestress effect. The finite element predictions were validated against the appropriate analytical solution proposed by Chell and experimental results.