Circadian rhythms are genetically determined biological rhythms that are considered an important adaptive mechanism to the cyclical light/dark alterations in the Earth environment. Age-related changes in the circadian time-keeping mechanism are well known, and seemingly contribute to various pathologies of aging. Recent findings demonstrate that the circadian system and circadian proteins play direct roles in many physiological processes, including those associated with aging. The core circadian proteins BMAL1 and PERIODs, in addition to their known functions in the circadian oscillator, play essential non-redundant roles in the control of tissue homeostasis and aging. Although the exact mechanisms are unknown, the involvement of circadian proteins in the regulation of metabolism, genotoxic stress response and reactive oxygen species (ROS) homeostasis can be responsible for the premature aging, observed in some circadian mutants. The understanding of the molecular mechanisms of these non-circadian activities of the circadian proteins will ultimately lead to the improvement in prevention and treatment of age-related pathologies.