Quantum well semiconductor structures allow small, fast, efficient optoelectronic devices such as optical modulators and switches. These are capable of logic themselves and have good potential for integration with electronic integrated circuits for parallel high speed interconnections. Devices can be made both in waveguides and two-dimensional parallel arrays. Working arrays of optical logic and memory devices have been demonstrated, to sizes as large as 2 048 elements, all externally accessible in parallel with free-space optics. This article gives an overview of the physics underlying the operation of such devices, and describes the principles of several of the device types, including self-electrooptic effect devices (SEEDs).