A general framework for the operation of a large class of quantum confined light modulators is offered, supported by a discussion of the theoretical background. The design of a quantum confined modulator for a specific host system allows a wide choice of modulation mechanism, geometrical design configuration and field-controlled optical parameter. In this short review, we discuss specific combinations of optical property, semiconductor material system and geometry suitable for selected digital and analogue applications. Excitonic optical effects are emphasised as providing electric-field control of both optical absorption and refractive index in multiple quantum well structures. Geometrical aspects are also discussed, including phase modulation in a horizontal waveguide configuration and absorption modulation in transmission and reflection-mode vertical modulators. Self electrooptic effect devices (SEEDs) figure prominently across the range of optoelectronic system components and are used to illustrate various modulator functions in lightwave logic operations and neural networks.