Han debuterte i 1956 med romanen Barbara, og utga sin første barnebok, Dieser Hund heißt Himmel, i 1978. Hans siste bok er en biografi om Wilhelm Tell.
Han mottok Deutscher Jugendliteraturpreis og Schweizer Jugendbuchpreis i 1996 for Als die Welt noch jung war.
Da han i 2008 ble tildelt H.C. Andersen-prisen for et samlet forfatterskap sa juryen at «the jury has recognized a very powerful narrator who fascinates the reader with playful reflection upon the creative process. His short philosophical stories are told through believable metaphors from a child's perspective and charm the reader. Humanity and universality are the most important characteristics of his works»
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