The Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL) is a space telescope and the fourth medium-class mission of the European Space Agency's Cosmic Vision programme. The mission is aimed at observing at least 1000 known exoplanets using the transit method, studying and characterising the planets' chemical composition and thermal structures. Compared to the James Webb Space Telescope, ARIEL will have more observing time available for planet characterisation but a much smaller telescope and it will be launched almost a decade later.