Recent studies, by a number of different biophysical methods, and in particular by electron-microscopy, have enabled a fairly simple hypothesis to be put forward to describe many features of the molecular basis of contraction in striated muscle. [J. biophys. biochem. Cytol. 3, 631 (1957); in “The Cell” (ed.: Brachet and Mirsky), Academic Press (in press); in “The structure and Function of Muscle (ed.: Bourne), Academic Press (in press)]. The evidence for and against this so called ”sliding-filament hypothesis“ was reviewed, and the relations between the physiological properties of muscles and the detailed molecular mechanisms involved in such a scheme were described. The advances in electron microscope techniques needed to take the structural studies a stage further were considered.