The authors introduce the full content of the Microscopic Theory of Superfluid He II, developed since 1998; also given are brief accounts of the application of one concept from the theory, the QCE1 Superfluidity Mechanism, to superconductors. One peer review report writes: The authors include more of the underlying physics than some earlier theories, and the comparisons they make with experimental data are satisfactory . The Microscopic Theory of Superfluid He II has several important features, which distinguishes this theory from the previous theories of He II. The immense volume of information the authors have today, especially the pieces of information revealing the microscopic dynamics of the system, was not available to the developers of the previous theories in the 1930s-1940s. This book also demonstrates how the general principles of quantum mechanics and condensed matter physics can be consistently applied to a given system with confidence, once a realistic microscopic model is derived for it. It demonstrates in turn the validity of the general physics principles in such an extreme system as the quantum fluid He II.