The aim of this book, written by a researcher at the Tatarstan Academy of Sciences, is to examine how and why theories change in science. Nugayev’s analysis, and his many examples, are confined to mathematically formalized theories of physics. Nugayev’s ideas are inspired by, and relate to, Russian scholars. His approach is primarily philosophical and clearly in the analytical tradition of Popper, Kuhn, Lakatos, Feyerabend, Stegmuller and others. Although Nugayev’s book is primarily addressed to philosophers, it is also of interest to the philosophically inclined historian of science.