The mechanisms of the biological clock are today being investigated in single neurons in cell culture or in unicellular and in other microorganisms. The results show that all components of this "endogenous clock" can be found at the cellular level. The cellular circadian program is controlled by a complex system of biochemical reactions, which can contain more than one circadian pacemaker and which comprises several feed-back loops at the input and the output side. This complex temporal program is a prerequisite for specialization and survival within the chrono-ecological niches of the "temporal space" day. It enables organisms on the one hand to adaptively react to environmental changes and thereby reaching transient independence of the external, physical time course; on the other side, it ensures that the endogenous day never runs out of synchrony with the solar day of the environment.