At Little Oakley, near Harwich, an interglacial deposit has been identified and mapped over a distance of ca. 1 km by means of boreholes and from temporary sections. The interglacial sediments are chiefly silts and sands, which occupy a large river channel (150-175 m wide) trending W.S.W.-E.N.E. The channel sediments are variously underlain by London Clay, Red Crag and another fluvial deposit, the `Oakley Gravel' (one of 15 newly defined lithostratigraphic units), against which they also abut on their northern margin. The channel occurs at an elevation of between 18 and 24 m O.D. and is thought, on the basis of clast lithology, to have been occupied by the pre-diversion Thames at a point immediately upstream of its confluence with the Medway. The interglacial deposits are rich in fossils, which indicate accumulation during the pre- and early temperate substages of an early Middle Pleistocene interglacial stage. Pollen spectra from the base of one borehole may possibly relate to the terminal phase of the preceding late-glacial period. The balance of the palaeontological evidence suggests correlation of the main sequence with the Cromerian sensu stricto. The essential facts leading to this conclusion are given here, but detailed discussions of the palaeobotany, vertebrates, molluscs and ostracods are given in a series of separate papers. This correlation gains some support from amino acid epimerization data from the shells of certain aquatic molluscs. Palaeomagnetic measurements, indicating normal geomagnetic polarity, are also consistent with this correlation. The relation of the Little Oakley sequence to the regional geology is discussed, and the palaeogeographic history of the ThamesMedway river systems in this area is briefly reviewed.