There is a renewed interest in the fundamentals of energy metabolism, yet most people base their understanding on the views of generalists expressed in elementary textbooks. New techniques that enable analysis of thousands of metabolites provide useful data, but do not themselves substitute for an understanding of the fundamentals of metabolism. While classical ideas of metabolism are also valuable, some earlier ideas have not withstood further investigation. This book presents a personal philosophy but rests on what is broadly accepted by metabolic biochemists over the past few decades.