One main topic in astronomy since the first discovery of an extra-solar planet near a sun-like star in 1995 is the formation and dynamical evolution of planets. Although up to now we have not detected any planet near the size of our Earth, numerous dynamical studies have been undertaken to search for the dynamically stable orbits in systems with large planets. Special emphasis here is orientated on the stability zones of celestial bodies moving in the so-called habitable zone (=HZ)1 around a star. Such studies are important, especially with regard to present (CoRoT) and forthcoming (like e.g. Kepler, Darwin, TPF) space missions that try to find a planet – similar to our Earth – where complex life could have developed. Since we know that the development of a biosphere and the further evolution to intelligent life-form is a process on a long time-scale, the long-term stability of such a planet is an important and necessary condition for the habitability of a planet.