In all of the devices discussed in previous chapters the dimensions of device structures were large compared to the wavelength of electrons in the device. When the dimensions of the structure are reduced to the point at which they are approaching the same order of magnitude as the electron wavelength some unique properties are observed. This is the case with a class of devices that have come to be known as “quantum well” devices, which feature very thin epitaxial layers of semiconductor material. This chapter will introduce the basic concepts of quantum wells and will describe some of the novel kinds of devices that can be made by using them. Improved lasers, photodiodes, modulators and switches can all be made by employing quantum well structures. Quantum well devices can be monolithically integrated with other optical and electronic devices to produce optical integrated circuits and opto-electronic integrated circuits.