In recent years there has been an increasing need for data on the rates of fatigue crack propagation in alloys of interest to the aircraft industry. In general alloys in which cracks grow slowly, under given stress conditions, have an obvious advantage over those in which cracks grow faster; there is more time to detect a crack before failure. It is possible that, in the future, quantitative crack propagation data may be used to give better estimates of the safe life of a structure. There is, however, a need to rationalise the presentation of such data and to study the effect of such parameters as specimen geometry and stress level. Some experiments to investigate the effect of varying some of these parameters in one clad aluminium alloy, DTD 5070A are described. It is a summary of work contained in two RAE Technical Reports.