This Classic Article is a reprint of the original work by Otto Heinrich Warburg, The Chemical Constitution of Respiration Ferment. An accompanying biographical sketch of Otto Heinrich Warburg, PhD, MD, is available at DOI 10.1007/s11999-010-1533-z . The Classic Article is from Warburg O. The chemical constitution of respiration ferment. Science. 1928;68:437-443. Reprinted with permission from AAAS.