Time, Clocks and Fundamental Physics

Time is the most basic notion in physics. Correspondingly, clocks are teh most basic tool for the exploration of physicsl laws. We show that most of the fundamental physical principles and laws valid in today´s description of physical phenomena are related to clocks. Clocks are an almost universal tool for exploring the fundamental structure of theories related to relativity. We describe this structure and give exmaples where violations of standard physics are predicted and, thus, may be important in the search for a theory of quantum garvity. After stressing the importance for future precise clock experiments to be performed in space, we refer to the OPTIS mission, to which another article in this issue is devoted. It is also outlined that clocks are not only importnat for fundamental tests but at teh same time are also indispensable for practical purposes like navigatio, Earth sciences, metrology, etc