The existence of defects in amorphous semiconductors is itself an problem of fundamental interest. Conventional approaches accept their existence only phenomenologically. We described a new approach to defining and understanding defects. The existence of defects is a natural outgrowth of our thermodynamical ensemble theory of electronic states in disordered systems. Specific properties are also predicted through properties of thermodynamic ensembles and the relationship between defect electronic properties and structural energies. Thus we obtain directly electronic properties without a need for detailed microscopic information. © 1987.