The Purbeck-Wealden of the type areas in onshore southern Britain (Fig. 1) encompasses the Berria-sian-earliest Aptian stages of the Cretaceous, a time span of approximately 21 Ma. At least some of the lowest part of the succession belongs to the latest Jurassic (Portlandian); how much depends on where the Jurassic-Cretaceous boundary is placed. Ostracods are frequently diverse and abundant in the predominantly calcareous or argillaceous units (e.g. Purbeck Limestone Group, Wadhurst Clay Formation, Grinstead Clay Formation, Weald Clay Group), sometimes forming ostracod limestones, but in clays that have undergone pedogenesis and in arenaceous facies (e.g. Ashdown Beds Formation, Upper and Lower Tunbridge Wells Sand formations) they tend to be rare or poorly preserved. They are essentially non-marine faunas, with only a few convincing indicators of direct marine influence. Although their interpretation is difficult and has given rise to controversy, ostracods are undoubtedly the most useful biostratigraphical tool available in Purbeck-Wealden sequences. They have also been used with some success in correlations of the offshore ‘ Purbeck-Wealden’ facies of Portlandian-Barremian age found in the Celtic Sea and Fastnet basins between southern Ireland and SW England (Fig. 1).