This article reviews some of the more important applications of semiconductor quantum wells in optoelectronic devices. The use of quantum wells has been beneficial for three main research directions. Firstly, improved performance has been obtained in devices such as laser diodes, light-emitting diodes and solar cells. Secondly, new devices with operating principles related to the two-dimensional quantum confinement have been invented; examples are the quantum-confined Stark effect modulators and logic gates, and intersubband infrared detectors. Finally, quantum effects have been studied in new systems, and elusive fundamental phenomena such as Bloch oscillations have been observed for the first time.