Combining engineering, computer, and photographic techniques with historical sources, this research note discusses the gas chambers attached to crematoriums at Auschwitz I and the Auschwitz-Birkenau death camp. Among other things, the authors identify the locations of several of the holes in the roofs through which Zyklon B was introduced: five in Crematorium I and three of the four in the badly damaged Crematorium II. The authors began their project before David Irving's libel suit against Penguin Books and Deborah Lipstadt, proceeding simultaneously with, but independently of, the trial. The defense presented the first version of the authors' report during Irving's subsequent application to appeal. Irving's application was rejected by the court.