The Alfvén Mission has been proposed by an international team to the ESA M5 competition and is currently under evaluation. The scientific aims of Alfvén are to explore particle acceleration processes and their consequences for electromagnetic radiation and energy transport in strongly magnetised plasmas. The mission will employ two spacecraft in the Earth's auroral acceleration region. Its findings will inform studies of similar processes in other strongly magnetised plasmas, from the Sun and other solar system planets to a variety of astrophysical radio sources.