The classical concept of the interaction of collectivised almost free valent electrons with the periodic field of a crystalline lattice forms the basis of the zone theory of metals. However, the successful development of this theory has not yet resulted in the solution of the problems of the origin of the actual alloys and compounds which have been produced experimentally by diffraction methods. This is one surprising difference between metal physics and quantum chemistry, where the structure of molecules is deduced from first principles -- the electron structure of the atoms and wave functions -- by applying chemical bond theory and the methods of valent bonds and molecular orbits. This book develops the concept of the metallic bond, uniting the ideas from metal physics concerning the collectivisation of electrons and chemical bond theory concerning the overlapping of orbitals.