Abstract Although the 8- N rule of covalent bonding is generally obeyed in amorphous semiconductors, well-defined defect centers exist and these control the electronic properties of the solids. The defects have two distinct reasons for their presence—they can arise from either strains upon material preparation or thermodynamic considerations. The strain-related defects characterize those amorphous solids in which the average coordination number is larger than approximately 2.4; their concentration is ordinarily very sensitive to the preparation techniques. In contrast, thermodynamically induced defects arise because of their low creation energy, and a minimum concentration characterizes any given material. These ideas have led to a resolution of several major puzzles with regard to the electronic properties of the two major classes of amorphous semiconductors—chalcogenide glasses and amorphous silicon-based alloys. Pure amorphous silicon is overconstrained and has large defect densities, but these can be reduced by many orders of magnitude if the material is alloyed with monovalent atoms such as hydrogen or fluorine. On the other hand, amorphous As 2Se 3 always contains a high defect density, for thermodynamic reasons. In addition to the concentration of defects present in a given material, its electronic properties depend critically also on the nature of these defects. In particular, the sign of the effective correlation energy of the defect with the lowest creation energy is of the utmost importance.