The circadian system in mammals is composed of a hierarchy of oscillators functioning at cellular, tissue, and systemic levels. The central suprachiasmatic nucleus (SCN) acts as a master pacemaker, driving rhythms in activity, rest, feeding, body temperature, and hormones. Coupling within the SCN network confers robustness to the SCN pacemaker, which provides stability to the overall temporal architecture of the organism. In contrast, cell autonomous circadian clocks are intimately integrated into metabolic pathways, suggesting their fundamental role in orchestrating metabolism. The SCN receives photic input from intrinsically photosensitive retinal ganglion cells (ipRGCs) and is composed of ~20,000 neurons, each containing a cell autonomous circadian oscillator. The SCN functions as a network where intercellular coupling ensures robustness and phase coordination. Peripheral oscillators, found in most cells and tissues, are also entrained by the SCN and can generate rhythmic behaviors in the absence of the SCN. These peripheral clocks are regulated by local and systemic cues, including hormone secretion, body temperature, and metabolic signals. The SCN controls peripheral oscillators through sympathetic and parasympathetic pathways, and peripheral oscillators can influence the SCN and other peripheral tissues. Food- and drug-sensitive oscillators (FEO and MASCO) can drive rhythmic outputs in the absence of the core molecular clock mechanism. The circadian system is highly interconnected, with outputs from one oscillator becoming inputs to others, and reciprocally influencing each other. This feedback loop ensures the temporal organization of various physiological processes and the integration of circadian rhythms with metabolic and other physiological systems.The circadian system in mammals is composed of a hierarchy of oscillators functioning at cellular, tissue, and systemic levels. The central suprachiasmatic nucleus (SCN) acts as a master pacemaker, driving rhythms in activity, rest, feeding, body temperature, and hormones. Coupling within the SCN network confers robustness to the SCN pacemaker, which provides stability to the overall temporal architecture of the organism. In contrast, cell autonomous circadian clocks are intimately integrated into metabolic pathways, suggesting their fundamental role in orchestrating metabolism. The SCN receives photic input from intrinsically photosensitive retinal ganglion cells (ipRGCs) and is composed of ~20,000 neurons, each containing a cell autonomous circadian oscillator. The SCN functions as a network where intercellular coupling ensures robustness and phase coordination. Peripheral oscillators, found in most cells and tissues, are also entrained by the SCN and can generate rhythmic behaviors in the absence of the SCN. These peripheral clocks are regulated by local and systemic cues, including hormone secretion, body temperature, and metabolic signals. The SCN controls peripheral oscillators through sympathetic and parasympathetic pathways, and peripheral oscillators can influence the SCN and other peripheral tissues. Food- and drug-sensitive oscillators (FEO and MASCO) can drive rhythmic outputs in the absence of the core molecular clock mechanism. The circadian system is highly interconnected, with outputs from one oscillator becoming inputs to others, and reciprocally influencing each other. This feedback loop ensures the temporal organization of various physiological processes and the integration of circadian rhythms with metabolic and other physiological systems.