PLATO (PLAnetary Transits and Oscilliations of stars) is a proposed mission of the European Space Agency's Science programme Cosmic Vision 2015–2025, currently under industrial study, and with a planned launch by the end of 2017. Its task is to better understand the properties of exoplanetary systems. As such it will detect and characterise exoplanets using their transit signature in front of a large sample of bright stars and simultaneously measuring the seismic oscillations of the parent star of these exoplanets. The mission will be orbiting the Sun-Earth L2 point, which provides a stable thermal environment and maximum uninterrupted observing efficiency. The payload consists of a number (> 28) of individual catadioptric telescopes, covering > 550 sq. degrees. Since the goal is to search for terrestrial exoplanets within the habitable zone of their host stars, and carry out asteroseismological observations of the host stars, very high photometric precision, high time resolution, and high duty-cycle visible photometry is required. Ground-based observations are needed to complement the observations made by PLATO to allow for further exoplanetary characterization. This paper consists of a summary of the preliminary results achieved by the ESA internal pre-assessment study.