This Memoir reviews, revises and interprets the biostratigraphy, lithostratigraphy, process sedimentology, palaeoenvironments and sequence stratigraphy of the classic Cambrian succession of SW Wales. This 1250m thick clastic sedimentary succession, of Terreneuvian to Furongian age, was deposited in environments ranging from alluvial fan to mid–outer shelf. Two lithostratigraphical groups are distinguished, each comprising six formations; where possible and appropriate, original and well-known names are retained in harmony with current usage. High-resolution sequence-stratigraphical techniques, constrained by biostratigraphical data whenever possible, allow the revised lithostratigraphy to be integrated with the western Avalonian chronostratigraphy developed in maritime Canada. A twofold subdivision is recognized. Megasequence 1 (sequences 3-7) spans the Terreneuvian, C2 and much of C3, with deposition occurring in an extensional rift-like regime on an epeirogenically active platform. Sequence 8 lies at the base of Megasequence 2, which extends from late C3 into the early Ordovician, and represents passive margin sedimentation.