This monograph brings together into a unifi.ed whole the ideas and problems that have been considered in many of my published works (articles in Voprosy filosofii and papers in collective works devoted to the philosophical problems of science, published in Russian, for instance, in the series Dialectical Materialism and Modern Science. Most of the problems or the aspects of them that are treated here have been discussed in my earlier work, but the present book con­ tains new material, additions, and more precise defi.nitions, and also a number of new conclusions. I have been especiallyinterested to show that dialectics and its very important requirement of applying the all-round universal flexibility of concepts objectively is the logic of modern science.The basic substance of the book (as its title says) is Marx­ist-Leninist dialectics in modern physics. Dialectics is not a formal mental construction but rather a living method of cognising nature and of searching for new truths in modern science, and in physics in particular, as far as this book is concerned .