Heterostructures, made by growing several ultrathin layers of different materials (down to a few atoms in thickness) on top of a silicon or other substrates, have remarkable properties not shared by bulk materials. One can, for example, confine the motions of electrons to a single layer, making it possible to investigate effectively two- dimensional systems. One can also build materials with large-scale periodicities by alternating layers of different compositions, thereby modulating the optical and electronic properties of the resulting structure. This graduate-level textbook provides the theoretical basis and the relevant experimental knowledge underlying our present understanding of the electrical and optical properties of