The experimental results pertaining to the electronic structure of covalent amorphous semiconductors are briefly reviewed. It is found that three classes of materials exist, depending on the lowest-energy coordination of the predominant chemical component. In each case, the transport properties are ordinarily controlled by the localized states in the gap resulting from the minimum-energy defect sites, in which the local coordination is not optimal for certain atoms. These localized states are treated in terms of a Hubbard model, in which the effective repulsion between two electrons simultaneously present on the same center is taken as positive for tetrahedrally bonded solids and negative for chalcogenide and pnictide glasses. The electronic structure is discussed in detail. It is shown that even such a simple model can account for almost all of the experimental properties of the major classes of amorphous semiconductors.