This survey was written at the invitation of the Editors of the Ergebnisse der Physiologie. Its aim is to present the more recent progress in the know ledge of biological energy transformations. Since it was intended for a review journal, the reader was taken to be familiar with the fundamentals of current biochemistry, as described in the standard textbooks. It was not the object to compile an extensive collection of facts. The survey is limited to aspects of wider interest, and the main emphasis has been on the general unifying principles which emerge from the great mass of detailed ob servations. The article is reprinted in the hope that it may be useful in this form to advanced students and research workers in biochemistry and related subjects. H. A. KREBS H. L. KORNBERG 2 Table of Contents Page 1. The Key Position of Adenosine Triphosphate . . . 213 2. The Three Phases of Foodstuff Degradation. . . . 213 3. The Energy-Yielding Steps of Intermediary Metabolism 215 4. The Build-up of Phosphate Bond Energy ..... 221 5. Alternative Pathways of Anaerobic Fermentation in Micro-organisms. 227 6. Alternative Pathways of Glucose Oxidation . 237 7. The Path of Carbon in Photosynthesis . . . 243 8. Utilization of Energy for Chemical Syntheses 249 9. Control of Energy-Supplying Processes . . . 262 10. A Special Feature of ATP as an Energy Store. 271 11. Evolution of Energy Transforming Mechanisms 273 Appendix by K. BURTON Free Energy Data of Biological Interest 27S References . . . . . . . . . . . . . . .