Experiments on a model material have shown that creep-fatigue interaction is possible by one of several mechanisms. At low temperatures failure occurs by initiation and propagation of a crack, and is transgranular. At higher temperatures failure is caused by cumulative damage of intergranular cavities dispersed throughout the specimen. At the transition a regime is found where failure occurs by initiation of an intergranular crack near the surface which propagates as a Stage II transgranular crack. In the cumulative damage mode of failure a further distinction is possible in terms of ‘r’ type and ‘w’ type cavities. Which one of them is dominant depends on the cycle shape. The behavior of the present aluminum alloy should be representative of those materials in which nucleation of cavities is difficult, such as austenitic stainless steels and precipitate strengthened nickel base alloys, but not of materials which cavitate rather easily,e.g., copper and oxide dispersion strengthened alloys.