This chapter presents a simplified manner some of the basic ideas of wave mechanics so as to give at least a plausible explanation of phenomena that cannot be explained by classical mechanics. It describes the foundation for understanding some of the important concepts related to electric conduction in semiconductors. A critical difference between metals and semiconductors is the much lower conductivity of semiconductors. In the presence of gradients of concentration and of electric potential, the fundamental concept of electrochemical potential provides a unifying and meaningful framework for understanding the behavior of semiconductors. The most important semiconductor materials at present are the crystalline forms of the element silicon and the compound gallium arsenide. A characteristic feature of semiconductors is that the current can be a drift current due to an applied electric field, as in metals, or a diffusion current due to differences in carrier concentrations in different parts of the semiconductor.