This book is different from numerous other textbooks and monographs about friction. In this work we study various effects and manifestations of friction in order to identify those properties of friction that are invariant for different friction mechanisms and thus constitute the very essence of friction as a physical phenomenon. By doing that we are making the case that friction is a fundamental force of nature whose properties can be deduced directly from the second law of thermodynamics, rather than treated as a collection of phenomenological unrelated effects and empirical facts-- Provided by publisher.