Industriously spreading his talents wide, Rasmussen was and is an important figure in present-day Danish music, as composer, writer, music historian, conductor, professor, administrator and organizer of festivals and concerts.

Concepts such as ‘music about music’ or ‘music on music’ were keys to his works from the 70s, as he often used pre-existing musical material in new connections and for new purposes, not as collage or quotation, but in a densely woven montage of idioms from widely different types of music. The piece Genklang for five pianists (1974) is an early model example. 

Montage technique and re-composing historical material also characterize Rasmussen’s later music, including music by composers as diverse as Vivaldi, Brahms, Satie, Carl Nielsen, Mahler, Charles Ives and others. He completed an unfinished opera by Schubert and a piano sonata by Schumann left incomplete, and his fragmented style was replaced by an increased interest in form and development. 

His several dramatic works for the stage and for radio, among them Majakovsky, Titanic and The Story of Jonah show the first signs of this change, which was continued in works such as A Symphony in Time and the string quartets Solos and Shadows and Surrounded by Scales

In the 80s he established The Danish Piano Theatre in an attempt to create a transportable and "poor" music theatre, far from all institutions. He made a cabaret-like theatre piece for two singers, mime, percussion, four-handed piano and sound technique and in a sense the piano became the protagonist (with  up to eight hands playing the same instrument!) The piano became a veritable "Musikkapelle", somewhere between historical time and rhythm box.

With the chamber symphony Movements on a Moving Line his music assumes the character of a labyrinthine ’weaving together’ of the same music in different tempi, and paired with new ideas about time and of tempo (partly related to so-called fractal mathematics) such considerations dominated his music for long. After the turn of the century psychological dimensions emerged, notably in the huge violin concerto Sinking Through the Dream Mirror, in Scherzo with Bells and Webs in a stolen Dream. And Rasmussen focussed particularly on the listener's experience of time in works such as A Tempo for orchestra (2001) and Postludes for 23 solo strings (2012). In recent years he has worked closely together with the baroque orchestra Concerto Copenhagen, resulting in works such as Concerto in Amber, Follia, follia, a concerto for baroque oboe, a baroque violin concerto and a "modern" version of Vivaldi's The Four Seasons. And several new recordings of his music have appeared.

Rasmussen is also a prolific writer of essays, so far collected in three volumes. And his many monographs, among them books on Glenn Gould, Sviatoslav Richter and a host of classical and contemporary composers, have reached a wide audience (and several have been translated). He also completed the vast Music in the 20th Century - a story and more recently published books on Haydn, Vivaldi and Buxtehude. And a monograph on Richard Strauss will appear soon. The span of his interests reaches from a monograph on John Cages music to a collection of conversations with million-selling Danish singer-songwriter Lars Lilholt. 
His music has been performed in more than 30 countries. Among several other awards he received the Carl Nielsen-Prize in 1991, the Wilhelm Hansen Composer Prize in 1997 and in 2017 the Frobenius Great Award of Honor.

Also an autobiography, The Devil's Fork, is upcoming.