The very low-grade to low-grade metamorphism is studied in the autochtonous tectonic units, the Helvetic and Prealpine nappes of the northwestern Alps (Savoy and Switzerland). It appears that the Mesozoic and Tertiary sediments of some of these units had been weakly metamorphosed during the Alpine Orogeny before the formation of the nappes, since it can be observed that more metamorphic units overthrust less-metamorphic ones. With regard to the Alpine Orogenesis, this implies that the sediments of the Helvetic and Prealpine nappes were variously buried (to maximum depth of more than 15 km) just before the overthrusting movements. Initial subductions engulfed sediments of the nappes to considerable depths, where they became metamorphic and were followed by uplift and gravity sliding. One compares the metamorphism of the external parts of the Alps with the stronger metamorphism of the Penninic Area, which is classically known as post-tectonic. Finally the tectonic evolution of the Alps from upper Cretaceous to present times is explained taking in account the significance of metamorphism.