Up until now, our discussion has been centered on the discovery and basic properties of conventional and high temperature superconductors. One of our aims is to give a simplified account of current understanding of the mechanisms giving rise to superconductivity; however, by its very nature this has to be rather theoretical in content. To set up an explanation for the origins of superconductivity, it is necessary to digress and provide some essential background material on quantum mechanics and explain how it has completely transformed our understanding of the behavior of electrons in atoms and normal metals. This is the purpose of the present chapter, which is therefore a diversion from the main theme. Many of the ideas described originated during the first thirty years of the twentieth century, one of the most exciting and important eras in the history of physics. The innovations and discoveries made during that time mark the transition from classical to modern physics.