[1] Frei's research work investigates how German society came to terms with Nazism and the Third Reich in the aftermath of World War II.
[2] From 1973 to 1978 Frei studied modern history, political and communication sciences in Munich, where he also completed editorial training at the German School of Journalism.
From 2005 to 2017, he was chairman of the Scientific Advisory Council of the Koebner Minerva Center for German History at the Hebrew University of Jerusalem.
Frei's longer-term research projects concerned the practice of reparation in Germany and Israel since 1952 and the history of the Flick Group in the Third Reich, both completed in 2009.
Larger ongoing research projects concern the history of political education in the Federal Republic and the experience of the long transformation in East Germany (1970-2010).
Frei participated in numerous contemporary historical debates and endeavors to convey scientific findings to a broader public.
In a 2015 television interview on the mainstream German ARD channel, Frei explained that the book contains a wealth of information about Auschwitz concentration camp and also Birkenau.
[10] In 1997 Frei investigated how the myth of the clean Wehrmacht was created through the official policies of the West German government led by Konrad Adenauer.