Book excerpt: Examines the basic electronic and optical properties of two- dimensional semiconductor heterostructures based on III-V and II-VI compounds. Explores various consequences of one-dimensional size-quantization on the most basic physical properties of heterolayers. Beginning with basic quantum mechanical properties of idealized quantum wells and superlattices, it discusses the occurrence of bound states when the heterostructure is imperfect or when it is shone with near bandgap light.