We describe a construction of the Lie group structure on the diffeomorphism group Diff(Rn), modelled on the space D(Rn,Rn) of Rn‐valued test functions on Rn, in John Milnor's setting of infinite‐dimensional Lie groups. New tools are introduced to simplify this task. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)