The hippocampal formation and parahippocampal region are prominent components of the rat nervous system and play a crucial role in learning, memory, and spatial navigation. Many new details regarding the entorhinal cortex have been discovered since the previous edition, and the growing interest in the area of CA2 has been covered in this chapter. Emphasis is on a conceptual change: instead of perceiving the hippocampal circuit as the standard sequential processing network, current insights favor the concept that multiple parallel networks are present. Many new facts, combined with a thorough restructuring of information and inclusion of pointers to relevant (online) resources, make this chapter relevant to both the novice and senior readership.