The article reviews the organization and mechanisms of circadian rhythms in diverse organisms, from unicellular to multicellular species. Circadian rhythms are universal in nature, governing daily cycles of biological activities. In unicellular organisms, stand-alone clocks generate 24-hour rhythms for various processes, while in multicellular organisms, clock functions are partitioned among different cell types to coordinate tissue-specific rhythms. The core of all circadian clocks is an internal autonomous oscillator that contains positive and negative elements forming autoregulatory feedback loops. These oscillators can be entrained by environmental cues, such as light and temperature, and can regulate downstream clock-controlled gene (CCG) expression and biological activity. The review discusses the complexity of circadian clocks in five model systems: cyanobacteria (*Synechococcus elongatus*), filamentous fungi (*Neurospora crassa*), fruit flies (*Drosophila melanogaster*), mammals, and birds. In cyanobacteria, a pacemaker based on the Kai clock genes orchestrates global rhythmic regulation. In *N. crassa*, multiple oscillators respond to different environmental inputs, including temperature and light, with some acting as pacemakers. In mammals, the suprachiasmatic nucleus (SCN) serves as a light-entrainable pacemaker that coordinates peripheral oscillators in various tissues. In birds, a more complex system involves pacemakers in the pineal gland, retina, and SCN, each contributing to different outputs. In *D. melanogaster*, a distributed set of autonomous oscillators with pacemaker function is present, allowing tissue-specific specialization. The article highlights the importance of understanding the coordination mechanisms between oscillators and the role of pacemakers in maintaining robust circadian rhythms. It also emphasizes the need for further research to identify and characterize key components of these oscillators to elucidate their functions and entrainment mechanisms.The article reviews the organization and mechanisms of circadian rhythms in diverse organisms, from unicellular to multicellular species. Circadian rhythms are universal in nature, governing daily cycles of biological activities. In unicellular organisms, stand-alone clocks generate 24-hour rhythms for various processes, while in multicellular organisms, clock functions are partitioned among different cell types to coordinate tissue-specific rhythms. The core of all circadian clocks is an internal autonomous oscillator that contains positive and negative elements forming autoregulatory feedback loops. These oscillators can be entrained by environmental cues, such as light and temperature, and can regulate downstream clock-controlled gene (CCG) expression and biological activity. The review discusses the complexity of circadian clocks in five model systems: cyanobacteria (*Synechococcus elongatus*), filamentous fungi (*Neurospora crassa*), fruit flies (*Drosophila melanogaster*), mammals, and birds. In cyanobacteria, a pacemaker based on the Kai clock genes orchestrates global rhythmic regulation. In *N. crassa*, multiple oscillators respond to different environmental inputs, including temperature and light, with some acting as pacemakers. In mammals, the suprachiasmatic nucleus (SCN) serves as a light-entrainable pacemaker that coordinates peripheral oscillators in various tissues. In birds, a more complex system involves pacemakers in the pineal gland, retina, and SCN, each contributing to different outputs. In *D. melanogaster*, a distributed set of autonomous oscillators with pacemaker function is present, allowing tissue-specific specialization. The article highlights the importance of understanding the coordination mechanisms between oscillators and the role of pacemakers in maintaining robust circadian rhythms. It also emphasizes the need for further research to identify and characterize key components of these oscillators to elucidate their functions and entrainment mechanisms.