In this decade we have seen a rapid increase in our understanding of the nature of extra-solar planetary systems. Space missions such as CoRoT and Kepler have confirmed that extrasolar planets are common. The detection of ‘super-Earth’ planets has expanded our planet inventory towards small, rocky planets. A large diversity of planets became evident, much wider than found in our Solar System. This diversity raises new questions to the nature of these planets and their formation and evolution processes, such as: What is the composition and internal structure of these planets? What is their atmospheric composition? And for the smallest planets: are they potentially habitable and how could we detect this? Constraining our understanding of the underlying processes requires first an improved knowledge of the basic planet parameters, hence their mean densities, atmospheres and their age. The talk will give an overview on our knowledge of terrestrial exoplanets and how we will explore their nature with future space missions, like CHEOPS, TESS and PLATO 2.0.