The superposition of high- and low-frequency loadings has caused numerous fatigue crack problems throughout the history of metal airframes. Fighter aircraft fuel tanks are subjected to the superposition of low-frequency (0.1 to 1 Hz) maneuver loads and high frequency (50 to 300 Hz) vibrations due to panel flutter and fuel slosh. This loading can lead to cracks, and leaks, in shorter times than can be predicted by using either load condition alone. This paper examines two methods for predicting fatigue lives under combined high- and low-frequency loadings. Tests of beam element specimens and a simulated fuel tank were used to examine prediction accuracy of both techniques. Results show that both techniques accurately predict the effects of combined load frequencies on life when adjusted to correlate predictions with the extreme (high- and low-) frequency data.