The prominent French historian Marc Bloch, co-founder of the Annales School, was one of the victims of Klaus Barbie. He was shot by the Gestapo on June 16, 1944. In his estate, an unfinished methodological work titled "Apologie der Geschichtswissenschaft oder Der Beruf des Historikers" was found - the first edition still bore the subtitle "How and Why a Historian Works." The work was originally published in 1949 by Lucien Febvre, who did not yet have all the manuscripts available and also made some alterations to the text. With this new edition, prepared by Etienne Bloch, the author's son, the complete and authentic text of this epochal work of modern historiography is now presented for the first time. It has been completely retranslated into German. This book offers valuable insights into the self-understanding and thought processes of modern historical research, into the goals but also the limits of historical knowledge, into methodology, and not least into the ethics of this "science in motion.".