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Fatigue crack growth of long and small cracks was investigated forhypoeutectic and eutectic Al-Si-Mg cast alloys. Crack growth behavior in the nearthresholdregime and Regions II and III was related to microstructural constituents namelyprimary α-Al dendrites and volume fraction and morphology of eutectic Si. Long cracksthresholds reflect combined closure effects of global residual stress andmicrostructure/roughness. The small crack threshold behavior is explained through closureindependent mechanisms, specifically through the barrier effects of characteristicmicrostructural features specific to each alloy. In Regions II and III changes in fracturesurface roughness are associated with different crack growth mechanisms at themicrostructural scale. The extent of the plastic zone ahead of the crack tip was successfullyused to explain the observed changes in crack growth mechanisms.