It is now accepted that exoplanets are ubiquitous. However little is known about those planets we have detected beyond the fact they exist and their location. For a minority, we know their weight, size and orbital parameters. For less than twenty, we have some clues about their atmospheric temperature and composition. How do we progress from here?We are still far from a hypothetical Hertzsprung–Russell diagram for planets and we do not even know whether there ever will be such classification for planets. The planetary parameters mass, radius and temperature alone do not explain the diversity revealed by current observations. The chemical composition of these planets is needed to trace back their formation history and evolution, as was the case for the Solar System.Pioneering results were obtained through transit spectroscopy with Hubble, Spitzer and ground-based facilities, enabling the detection of ionic, atomic and molecular species and of the planet’s thermal structure. With the arrival of improved or dedicated instruments in the coming decade, planetary science will expand beyond the narrow boundaries of our Solar System to encompass our whole Galaxy.In the next five years, ExoLights will address the following fundamental questions:– Why are exoplanets as they are?– What are the causes for the observed diversity?– Can their formation history be traced back from their current composition and evolution?New spectroscopic observations of a select sample of exoplanets’ atmospheres (~ 20 out of the 150 observable today) will be analysed with state-of-the art statistical techniques and interpreted through a comprehensive set of spectral retrieval models, developed by the PI and her team. This programme, together with the homogeneous re-analysis of archive observations of a larger sample of exoplanets, will allow us to use the chemical composition as a powerful diagnostic of the history, formation mechanisms and evolution of gaseous and rocky exoplanets.