Clocks in space missions

The status of present day physics which essentially is encoded in the Einstein Equivalence Principle is reviewed. This includes the Universality of Free Fall, the Universality of the Gravitational Redshift and the local validity of Lorentz invariance. This implies that gravity is a metric theory. This metric can be explored by positioning of freely falling objects or by clocks. The need of a theory of quantum gravity implies small deviations from this standard physics picture so that, e.g., different clocks are expected to behave differently in gravitational field. Furthermore, there are some observations which until now did not found a convincing explanation and may suggest that gravity should be modified at large distances. Clocks play an important role in space explorations of such modifications.