The mechanism by which electrons move in crystalline semiconductors is now understood in some detail. The theoretical concepts necessary for the understanding of conductions in non-crystalline materials involve new ideas, which are discussed in this article. These include ‘localized states’ and ‘thermally activated hopping’. Some experiments designed to test the theory are described. Certain switching devices which make use of the electrical properties of non-crystalline materials are described, and theoretical explanations are suggested.