Born 7 June 1939 in Halle (Saale); married. Graduated from grammar school in Marl (Westfalen). Studied law in Heidelberg and at the Free University of Berlin. Has practiced law since 1969 in Berlin. He was a member of the "Socialist Lawyers' Collective" for ten years, and has defended political criminals for thirty years, including members of the terrorist Red Army Faction.
In the late 1960s he was involved in the student movement. From 1970 to 1974, he was a member of the SPD. He co-founded the "Alternative List for Democracy and Environmental Protection," a predecessor to the Berlin chapter of the Greens. He was a member of the Bundestag from 31 March 1985 until 1987 and has been a member again since 1998. In 2002, he became the first Green politician ever to be directly elected to the Bundestag when he received a plurality of the votes in the Friedrichshain-Kreuzberg district. He was reelected in 2005. He is one of the vice-leaders of the Green caucus in the Bundestag and among the most prominent politicians belonging to the "Fundi" wing of the party.
He is the nephew of the popular former German football commentator Herbert Zimmermann.