Muscle contraction occurs when the thin actin and thick myosin filaments slide past each other. It is generally assumed that this process is driven by cross-bridges which extend from the myosin filaments and cyclically interact with the actin filaments as ATP is hydrolysed. Current biochemical studies suggest that the myosin cross-bridge exists in two main conformations. In one conformation, which occurs in the absence of MgATP, the cross-bridge binds very tightly to actin and detaches very slowly. When all the cross-bridges are bound in this way, the muscle is in rigor and extremely resistant to stretch. The second conformation is induced by the binding of MgATP. In this conformation the cross-bridge binds weakly to actin and attaches and detaches so rapidly that it can slip from actin site to actin site, offering very little resistance to stretch. During ATP hydrolysis by isolated actin and myosin in solution, the cross-bridge cycles back and forth between the weak-binding and strong-binding conformations. Assuming a close correlation between the behaviour of isolated proteins in solution and the cross-bridge action in muscle, Eisenberg and Greene have developed a model for cross-bridge action where, in the fixed filament lattice in muscle, the transition from the weak-binding to the strong-binding conformation causes the elastic cross-bridge to become deformed and exert a positive force, while the transition back to the weak-binding conformation upon binding of MgATP, causes deformation which, during fibre shortening, leads to rapid detachment of the cross-bridge and its re-attachment to a new actin site. From the results of in vitro experiments, it was furthermore suggested that relaxation occurs when the transition from the weak-binding to the strong-binding conformation is blocked. Results of recent mechanical and X-ray diffraction experiments on skinned fibre preparations are consistent with the assumed close correlation between the behaviour of isolated proteins in solution and the behaviour of cross-bridges in muscle. Furthermore, X-ray diffraction experiments allowed to provide experimental evidence for the postulated structural difference between attached weak-binding and attached strong-binding cross-bridges. Finally, recent studies have confirmed the prediction of Eisenberg and Greene that the rate limiting step in vitro determines the rate of force generation in muscle.