The hippocampus is critically involved in certain kinds of memory. During memory formation, it may operate as an integrated unit, or isolated parts may be responsible for different functions. Recent evidence suggests that the hippocampus is functionally differentiated along its dorsoventral (septotemporal) axis. The cortical and subcortical connections of the dorsal and ventral hippocampus are different, with information derived from the sensory cortices entering mainly in the dorsal two-thirds or three-quarters of the dentate gyrus. Rats can acquire a spatial navigation task if small tissue blocks are spared within this region, but equally large blocks at the ventral end are not capable of supporting spatial learning. In primates, the posterior hippocampus (corresponding to the dorsal hippocampus of rodents) appears to be more important than anterior areas for encoding of spatial memory and certain forms of nonspatial memory. The ventral (or anterior) hippocampal formation is to some extent disconnected from the rest of the structure both in terms of intrahippocampal and extrahippocampal connections and may be performing functions that are qualitatively different from, and independent of, those of the dorsal hippocampal formation.