A study has been made of creep damge accumulation in two casts of l%.Cr-1/2%i.Mo low alloy steel. Creep tests and creep crack growth tests have been carried out at 823K to determine the nature of the damage accumulation and to attempt to relate microscopic damage mechanisms to the macroscopic fracture parameters. Four types of specimen were tested and failure of all occurred by the continuous nucleation. growth and coalescence of grain boundary cavities. A mechanism for the growth of cavities is suggested. based on grain boundary diffusion coupled with geometric constraint. The influence of continuous cavity nucleation has also been considered and it is suggested that this phenomenon initially increases the rate of diffusive cavity growth. However continuous nucleation decreases the growth rate once the latter becomes constrained. The effect of stress-state is also considered and increasing triaxiality is shown to have little effect on the unconstrained diffusive growth but it decreases the constrained growth rate by increasing the overall constraint in the specimen. Predicted growth rates give good agreement to those observed experimentally for both notched and un-notched creep specimens. Reasonable agreement is also observed to the predicted rupture lives although the predictions suggest notch strengthening whilst experimentally notch weakening is observed. This is thought to be due to non-uniform damage formation on loading. Based on the above concepts of cavity growth, constitutive equations are presented to predict the time dependence of creep strain. These are found to give good agreement to the experimentally determined strain rates, lending further support for the development of continuum damage mechanics as a means of assessing creep crack growth behaviour.