The Nazi trials in the Federal Republic were a forum in which the crimes of National Socialism were tried as early as the early post-war period. The Holocaust survivors and former concentration camp inmates had a particularly controversial task as witnesses, which, however, has so far received little attention from researchers. In many cases it was up to them alone to convict the accused with their reports. At the same time, they were exposed to massive distrust from the German judiciary, which considered the survivors too biased to make objective assessments. The interrogations and confrontations with the perpetrators also presented a high burden. Nevertheless, thousands of survivors gave testimony of their own free will and endured the hardships to advance the prosecution of the crimes. Using the example of four Auschwitz trials from three decades, Katharina Stengel examines the importance of the victims for the Nazi trials, how the lawyers dealt with them and their incomprehensible reports, how the witnesses themselves acted in court, what concerns they pursued and what conclusions they drew from their experiences.