Using a direct proportionality between the rate of fatigue crack growth and crack opening displacement above a threshold, it is shown that fatigue crack growth data for a wide variety of different materials can be accurately described in terms of the mechanical properties and two material constants ; the constant of proportionality A and the threshold stress intensity factor Kth. Some 65 sets of data for tests at R ~ 0 were analysed by computer and it is shown that the approach is valid to growth rates up to about 10 .4 in]cycle, i.e. until the onset of crack propagation by dimple formation. It is found that A can be related to the yield strain for crack growth in non-aggressive environments, and is in r creased by increasingly severe environments, while Kt~ is decreased. These changes provide a measure of the severity of the environment. Crack growth rate in non-aggressive environments is shown to be independent of the yield stress and proportional to the strain intensity factor above the threshold. The tabulation of A and Kth values as a function of material, environment and loading conditions provides a systematic engineering approach to estimating rates of fatigue crack growth and in determining the residual lifetimes of flawed structures.