The flysch sequences of the Swiss Prealps (Alpine front ranges) are described and interpreted in the light of a decade of study by a group research project at Fribourg. The geographical and geological settings are illustrated. The stratigraphy and sedimentology, the sedimentary petrography and the geodynamic setting are discussed. Correlations within and outside the Prealps, as well as outstanding problems, are treated. The flysch units and sequences covered are the Gurnigel Flysch, the Médianes Flysch, Helminthoid Flysch, Fouyet, Manche and Mocausa Flysch, Hundsrück Flysch, Niesen Flysch and the Ultrahelvetic Flysch. The data from clast, framework-grain and heavy mineral analysis, from micropalaeontology, from facies analysis, palaeocurrent measurements and stratigraphy allow the interpretation of sedimentary environments, source terranes and depositional models. In all cases, flysch deposits are deep marine detrital accumulations and source areas were undergoing active tectonics. Individual models are based on the deep-sea fan concept, but interpretations tend to show significant modifications with respect to simple fan characteristics. These departures are to some extent due to available grain size, to active structuring of basins or to minor controls such as eustatic variations or climatic influence. The integration of sedimentological data with stratigraphy and regional structure furnishes palinspastic reconstructions in map view. These provide insights on the palaeogeography of the Western Alps from Mid-Cretaceous to Oligocene and give a picture of the evolution of the Western Mediterranean Tethys during the same period. The relationship between flysch deposition and alpine mountain building is confirmed, while constraints on basin evolution are established.