He attended and later lectured at the Darmstädter Ferienkurse (Darmstadt music summer schools).
Furthermore, he assisted other composers, including Mauricio Kagel, Franco Evangelisti, György Ligeti (Artikulation), Herbert Brün and Karlheinz Stockhausen (with the realization of Gesang der Jünglinge and Kontakte).
In 1964 Koenig moved to the Netherlands, where he taught at Utrecht University and was, until 1986, director and later chairman of the electronic music studio, which became the Institute of Sonology.
Among his notable students are Jorge Antunes, Mario Bertoncini, Konrad Boehmer, Karl Gottfried Brunotte, Miguel Ángel Coria, Johannes Fritsch, Annea Lockwood, Luca Lombardi, Tomás Marco, Pierre Mariétan, Zoltán Pongrácz, Kees van Prooijen, Atli Heimir Sveinsson, Claude Vivier and Jan Vriend.
His works Terminus 2 and Funktion Grün were selected by the British magazine The Wire in 1998 for its list of 100 Records That Set the World on Fire (While No One Was Listening) [de].