Memory function is centrally regulated by the function of the hippocampus. The activity of hippocampal neurons is modulated by several neurotransmitter systems including glutamatergic neurons from the entorhinal cortex, serotonergic fibers from the raphe nucleus, dopaminergic neurons from the ventral tegmental area (VTA), histaminergic cells from the tuberomammillary nucleus and adrenergic fibers from the locus coeruleus (LC). The complex interactions between hippocampal neuronal pathways contribute to our memory function. We we learn, retain and recal information, an army of neurons each sending or receiving chemical signals are engaged. This concise book touches on some of these complex interactions.