Three main problems in the energetics of muscular contraction confront us at the present time:- (1) Thermodynamics. This is a labyrinthine subject in the sense that there are often several equally correct ways (that may appear different) of arriving at the same result. The recent advances in the kinetics of the reactions of muscle proteins have increased still further the need for understanding among muscle physiologists-and other biologists-of those parts of thermodynamics that concern them directly, notably those relating work and chemical change. (2) Energy balance. Over the whole cycle of contraction, relaxation and oxidative recovery, the observed energy output (heat + work) can be accounted for by the chemical processes currently believed to operate in the cycle. In contrast, during contraction and relaxation alone, the energy output is greater than can be accounted for by the extent of known chemical reactions combined with their heats of reaction (measured calorimetrically). The physical and chemical methods cross-check one another-hence the usefulness of the method. The details of this discrepancy may vary from species to species but it is seen in all the experiments known at this time. The general problem will be resolved only when we know more about the chemical reactions that have occurred and reliable measurements have been made of their heats of reaction in vitro. (3) Work balance. Work and heat differ thermodynamically and, since the production of work is a primary function of muscles, the question naturally arises whether the observed splitting of ATP (combined with estimates of the change in free energy of this reaction) is sufficient to account for the work observed. On present information, which is imperfect in several respects, the chemical change is sufficient to account for the work. Estimates of the efficiency of transduction vary in different laboratories from 45 to 66%.