Circadian rhythms in mammals are generated by endogenous neural oscillating systems entrained to the light-dark cycle by specific visual pathways. We conclude from available data that the suprachiasmatic hypothalamic nuclei (SCN) are the principal circadian oscillators in the rodent brain and that a retinohypothalamic projection terminating in the SCN is the primary visual pathway subserving entrainment of circadian rhythms. Recent anatomical studies demonstrate that the SCN have distinct subdivisions in the rat. A dorsomedial component is comprised of a distinct neuronal population and contains a large population of interneurons, many of which produce peptides. It receives no direct or indirect visual input and has only very limited projections outside the SCN. A ventrolateral component is also made up of a distinctive neuronal population, receives both direct and indirect visual projections, and provides the major external projections of the SCN, which are to the hypothalamus, particularly the hypophysiotrophic area. The SCN are viewed in this review as containing multiple, mutually coupled oscillating systems that arise from a developmental process of interconnecting individual neuronal circadian oscillators into circuits that form the oscillating systems. A model for the organization of the systems is presented.