Harmonic generation of surface acoustic waves (SAW) is a useful tool for studying surface microcrack initiation during fatigue of high strength aluminum alloys. We have developed a model which relates the length and density of such microcracks, initiated during fatigue, with SAW harmonic generation signals. It was found that the resulting quantitative relationship between the acoustic data and the remaining fatigue life is accurate to within 5% over the last 50% of the fatigue life, spent in the microcrack initiation phase. Extensive tests are now underway to study the general applicability of these concepts to a wide range of high-strength Al alloys. Furthermore, we have started to separate the various contributions to harmonic generation from microcracks that are smaller than the average grain diameter, from those that have been able to grow out of the grain of their origin. These considerations are important in that they (1) provide detailed insight into the process of fatigue damage and (2) may improve our remaining life prediction capability, particularly during the first 90% of the total fatigue life. This concept seems to have applicability to the "Retirement for Cause" program of the Air Force.