Summary Muscle contracts by the cyclical interaction between two proteins, actin and myosin. The energy source for this interaction is ATP. Many approaches are used to understand the molecular basis of how this interaction occurs and how is it driven energetically by ATP, and each approach gives unique information. In this chapter a brief outline of each of the approaches used to understand muscle structure, mechanics biochemistry, and energetics was given, and how they contribute to an overall picture of how muscle works was discussed. The earliest ideas about muscle used the analogy of a spring; these were gradually revised to give rise to a sophisticated model in which steps in the ATPase cycle alter the binding affinity of the crossbridge to actin. A change in binding strength from weak to strong may correspond with some overall change in angle in the crossbridge as it generates force or motion. The newly presented crystal structures of myosin and actin show at the molecular level what these conformational changes might be and should lead to a definition of these changes in detail.