Animals, human beings among them, can adapt their behavior to (predictable) temporal fluctuations in the environment (such as day and night alternation, food and water availability, or social contact) by learning and memory processes interacting with an endogenous circadian clock. Behavioral, physiological, and biochemical circadian rhythms are crucial for the good mental health of an individual and rely on the integrity and functioning of the circadian system. The master clock in the suprachiasmatic nucleus synchronizes independent circadian peripheral clocks, localized in other brain areas, organs, and tissues to the appropriate phases by neural and humoral signals. Thus, circadian clocks orchestrate interactions between the organism's internal processes and the environment in healthy, but also in pathological, conditions. This chapter focuses on the main components of the circadian system; the temporal organization of cognitive functions at the molecular, biochemical, and behavioural levels and their clock-mediated regulation; as well as on factors that could disrupt the normal functioning of the circadian system and thus, contribute to the etiology of cognitive disorders.