The discovery of extrasolar planets was one of the major advances of the last two decades. Over 400 planets have now been detected and astronomers are beginning to characterise their composition and physical characteristics. To do this requires huge quantities of spectroscopic data most of which is not available from laboratory studies. The interdisciplinary ExoMol project will provide acomprehensive solution to this problem by providing spectroscopic data on all the molecular transitions of importance in the atmospheres of exoplanets. This data will be widely applicable to other problems and will be used for studies on cool stars, brown dwarfs and circumstellar environments. ExoMol will also be used by scientists who study spectraof hot molecules in a other situations such as combustions.A mixture of first principles and empirically tuned quantum mechanical methods will be used to compute comprehensive and huge (up to 100 billiontransitions) line lists. Novel methodologies will be developed for treating larger molecules such as methane and nitric acid. The success of ExoMolwill rely on these developments and the use of state-of-the-art computing.The ExoMol database will from part of the EU Virtual Atomic and Molecular Data Centre (VAMDC). A program to promote application of the data by a wide variety of users will be initiated.