Having celebrated its twenty-second anniversary, molecular beam epitaxy (MBE) has reached a very mature state as a crystallographic growth technique for a variety of semiconductor materials. It is widely used at universities, for industrial research in development laboratories and in production. Companies currently sell devices made from MBE-grown materials such as high-electron-mobility transistors (HEMTs), GaAs/AlGaAs lasers, photodetectors etc. The success of MBE technology can be explained by some of its inherent strengths. It uses extremely high-purity source material inside a vacuum chamber, making its operation predictable and safe. The growth process is relatively simple and can be monitored by in situ analysis tools. MBE played a key role in the exploration of new material systems such as GaAs on silicon and strained layers. Today's technology, together with the immense knowledge and understanding of growth aspects, makes it possible to achieve ultrathin layers, abrupt heterointerfaces and three-dimensional band gap engineering by growth on non-planar or tilted substrates. Areas in the optoelectronic field in which MBE has made substantial contributions are therefore III-V layers on silicon, including photodetectors and lasers, ultralow threshold and very high-power Ga(In)As/AlGaAs lasers, surface-emitting lasers which require very precisely controlled and stable fluxes, long-wavelength photodetectors, non-planar substrate devices etc. Areas that are more difficult to explore with MBE but are nevertheless very challenging include selective area growth and material systems from groups other than III and V which contain gallium, aluminum, indium and arsenic. Furthermore, newer fields such as low-dimensional structures with all kinds of in situ formations as well as further improvement of the MBE technology, including better diagnostic tools and group V flux controls etc., will engage the MBE community for several years to come. © 1991.