The Willunga Embayment of the St. Vincent Basin is one of the classic areas of Cainozoic geology in South Australia with published studies dating back over the past century. This present study reassesses the geology of the Middle Eocene to Early Miocene succession in this region in the light of recent extensive drilling. The stratigraphic units recognised here, from youngest to oldest are: Port Willunga Formation (Early Miocene to Late Eocene), including the Pirramimma Sand Member (Early Miocene to ?Late Eocene), the Ruwarung Member (Oligocene to latest Eocene) and the Aldinga Member (Late Eocene); Chinaman Gully Formation (Late Eocene); Blanche Point Formation (Late Eocene), including the Gull Rock Member; Tortachilla Limestone (early Late Eocene); and the Maslin Sands (?Late to Middle Eocene), comprising the South Maslin Sand Member (?Late to Middle Eocene), and the North Maslin Sand Member (Middle Eocene). This Eocene-Miocene succession thickens towards the southwest to a maximum recorded thickness of 365m. The principal marine units, the Port Willunga and Blanche Point Formations, are calcareous at the western end of the embayment and richly fossiliferous. To the east, towards the margin of marine deposition, non-calcareous sediments predominate. Within the Port Willunga Formation, the marginal-marine quartzose sandy facies is referred to the Pirramimma Sand Member. Two conspicuous intervals of siliceous sediments are recognised and described in the Willunga Embayment; the Gull Rock Member within the Blanche Point Formation, and the Ruwarung Member within the Port Willunga Formation. The North Maslin Sand and South Maslin Sand Members are of variable thickness in the embayment. The units interfinger at several localities and are grouped together as the Maslin Sands. Foraminifera are used to correlate the principal borehole sections. An aggregate range chart for the dianostic species is provided in addition to micropalaeontological logs for five bores. A substantial break in the sedimentary record, including the Chinaman Gully Formation and most of the Aldinga Member is recorded in the southwestern corner of the embayment. Sections in other regions of the St. Vincent Basin, viz. the Northern Adelaide Plains, Inkerman-Balaklava Coalfield, Yorke Peninsula and the Condowie Plains, are correlated with the succession in the Willunga Embayment.