This small book is a simplified, abbreviated, and updated version of the author's Free Energy Transduction in Biology, published in 1977 (Academic Press, New York). The present book is meant to be a textbook for a class or for self-study. The first chapter gives a self-contained and elementary discussion of the principles of free energy transduction in biology. Section 5 includes new material on the Onsager coefficients Lij (for systems near equilibrium) not avail­ able in 1977. Some readers may wish to study the first chapter only. The second chapteris a little more sophisticated, and deals with the so-called diagram method for calculating steady-state probabili­ ties and cycle fluxes. Although these concepts are useful in the analysis of free energy transduction systems, they have an intrinsic importance and interest. Section 8 summarizes quite recent new results not included in the 1977 book. The third chapter is again a step more sophisticated. Some readers may wish to omit it. Free energy levels of the states in a kinetic diagram are introduced. This topic is primarily of conceptual inter­ est for ordinary kinetic diagrams but it is essential in understanding muscle contraction (and related systems) at the molecular level.