Abstract There are three ways in which we can gain knowledge of the change of protein structure connected with muscular contraction. The first way is the observation of the contracting muscle itself. The changes of structural qualities and their time sequence have to be investigated. I take it that the excellent information which we have at present concerning the time sequence of mechanical, optical and energetic changes is one of the principal reasons for this meeting. Secondly to explain these phenomena we have to know also their localization. Up to the present we only have evidence that double refraction and probably also the changes of double refraction take place in the A band. The active change of length and tension in contraction may be also attributed to it. Thirdly, we must know what really happens to the single particles of muscle proteins at the respective points and times. That is to say, we must know the properties and reactions of the purified proteins. Along this third way a large number of new facts have been established recently. I consider it to be my task to-day to report on the most important of these facts, emphasizing those which could throw light upon the structure and the properties of the living muscle. Consequently I will not speak on the ‘corpuscular’ proteins of muscle, the myogen- and globulin-x fractions.