This is a personal account of some of the successive steps in our understanding of the structural mechanism of muscle contraction during the last 45 years. It describes how I, as an ex-physicist, came to be studying muscle by X-ray diffraction in 1949; how the concepts of the double array of actin and myosin filaments and, later, the overlapping filament model and the sliding filament mechanism were developed; and how further electron microscope findings of the structural polarity of muscle filaments led to the suggestion that analogous structures and mechanisms might be involved in cellular motility. The article describes briefly how synchrotron radiation has made it possible to obtain detailed structural information about contracting muscle with millisecond time resolution and discusses some of the recent major advances in the field and the prospects of reaching a full understanding of the contraction mechanism.