The hippocampal formation is a phylogenetically primitive cortical complex located in the temporal lobe of humans and in the caudal portion of rodent forebrain. This structure is essential for the formation of new declarative and episodic memories, and it also has a well-characterized role in spatial navigation. This chapter introduces the main neuronal types (principal cells and interneurons) of the hippocampal formation (hippocampus, dentate gyrus, and subiculum) and places those cells in a neuroanatomical, neurochemical, neurophysiological, and overall functional context. The principal neurons of the hippocampus, dentate gyrus, and closely associated entorhinal cortex form a relatively simple cross-sectional trisynaptic neurocircuit. Study of this functional neurocircuit led to the discovery of long-term potentiation, and continues to drive new understanding of the cellular and molecular mechanisms of neuroplasticity. Given the extensive study of stress effects on hippocampal function and neuronal plasticity,