Publisher Summary The hippocampal region is a prominent component of the mammalian nervous system, which has attracted the attention of neuroscientists since the beginning of formal study of the nervous system. Much of the available information concerning the cellular organization and intrinsic connectivity of the hippocampal region has been obtained from studies in the rat. This chapter provides an overview of the constituent regions, cytoarchitectonic organization, connectivity, and chemoanatomy of the mouse hippocampal region. The hippocampal region is taken to include two sets of cortical structures: the hippocampal formation on the one hand and the parahippocampal region on the other. The defining differences between the two are the number of cortical layers present and the overall principles of connectivity. Their connectivity is largely unidirectional, although recent findings in rats indicate that this may be an oversimplification. The hippocampal formation of the mouse appears grossly as an elongated structure, with its long axis extending in a comma-shaped fashion from the septal nuclei of the basal forebrain rostrodorsally, over and behind the diencephalon into the temporal portions of the brain caudoventrally. Differences are present, such as those observed in the terminal distribution of the perforant pathway in the molecular layer of the dentate gyrus and the dentate associational/commissural pathway terminal distribution. Some of the fundamental rules that govern the organization of the connectome of the hippocampus in the mouse are comparable to those observed in the rat, and this likely holds true for the primate hippocampus as well.