The discovery of the transition “ordering-phase separation”, which occurs in alloys when the sign of the chemical interaction between neighboring atoms A and B changes with temperature, has shown that a whole direction in science, namely Materials Science, has not fully moved on from the ideas of the previous century. The theories about the nature of alloys which have been implanted in us as students have turned out not to be correct enough to explain the processes present in alloys under thermal influence. In fact, these processes are determined only by the chemical interaction between nearest atoms A and B. Therefore, this book is the first publication to present exhaustive experimental and theoretical evidence proving the validity of this thesis for alloys. This radically changes our outlook on the nature of alloys, and makes it possible to rationalize the technology of heat treatment of alloys, compelling us to address the correction of existing phase diagrams and to use a conscious approach to the design of new alloys instead of the empirical one.