The science of exoplanets is one of the most rapidly changing and exciting areas of astrophysics. Since 1995, the number of planets around other stars has increased by three orders of magnitude. The smallest exoplanet known today is Mars-sized. The observation of the exoplanet atmospheres is now right at the cutting edge of exoplanet science. Using Hubble, Spitzer and ground-based telescopes, molecules such as water, methane and carbon-dioxide, have been discovered in the atmosphere of an exoplanet. In the next decade, the ESA mission candidate EChO (Exoplanet Characterisation Observatory) is expected to unveil the composition of tens of exoplanets, including potentially habitable ones.