The hippocampus is made up of physically distinct subfields, each with their own architecture, cell features, connectivity, electrical properties, and susceptibility to damage. The structure is divided into two main regions: the dentate gyrus (DG) and the cornu ammonis (CA), commonly known as Ammon’s horn. The CA region is separated into four different areas: CA1, CA2, CA3, and CA4. The term “CA” is derived from CA. The dentate gyrus (DG) is shaped like a “V” or “U,” but the cornu ammonis (CA) is a curved “U” structure located inside the DG. The most central portion of the DG is known as the hilus or hilar region. The main stratum of the DG, also known as the granule cell layer or stratum granulosum, contains granule cell nuclei. The granule and pyramidal cell layers align with the hippocampus’s septotemporal axis. Both the DG and the CA have a tiered structure. The DG is made up of three layers: the polymorphic layer, stratum granulosum, and stratum moleculare, which are ordered from inside out. The polymorphic layer is located within the hilus. The layers of the CA region, ordered from innermost to outermost, are stratum moleculare, stratum lacunosum (or lacunosum-moleculare), stratum radiatum, stratum lucidum (found only in CA3), stratum pyramidale, stratum oriens, and stratum alveus. The DG and CA contain two primary layers: stratum granulosum and stratum pyramidale. These layers contain the cell bodies of pyramidal and granule cells, respectively. The polymorphic layer of the DG, as well as the stratum oriens and radiatum of the CA regions, includes a variety of interneurons, such as mossy cells, pyramidal basket cells, axo-axonic cells (chandelier cells), fusiform cells, O-LM cells, IS neurons, the MOPP cell, and bistratified cells. The subiculum, presubiculum, parasubiculum, and entorhinal cortex are also important components in hippocampal development.