[en] Testing revealed that sulfur content and sulfur morphology are the most important variables determining the susceptibility of low alloy pressure vessel steels (SA508C and SA533B) to environmentally enhanced fatigue crack growth. The next most significant engineering variables are loading frequency and temperature. As each applied stress intensity factor level increased during crack growth tests, the frequency at which environmentally assisted cracking (EAC) occurred decreased. EAC was most pronounced at 200 degrees C, with the effect over a wide range in frequency. A host of other variables exert some influence on the existence of EAC, including oxygen content and R-ratio of the loading wave form. Sufficient information now exists regarding the magnitude and controlling variables for EAC so that, in many cases, realistic predictions of crack growth rates can be made. Overall, the research described in this report contributes substantially to the scientific understanding of EAC under fatigue loading conditions. Volume 1 of this report summarizes key results from the fatigue crack growth studies. Volume 2 provides details of the test methods and data from the studies