After his final degree, Sommer worked for the trade union. He climbed the ladder in the trade union becoming chairman of ver.di on 18 March 2001. A year later, on 28 May 2002, he was elected chairman of the DGB. He served for twelve years, retiring in 2014.[2] In 2010, he was also elected president of the International Trade Union Confederation (ITUC).
Sommer died in Berlin on 30 June 2025, at the age of 73.[3]
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