Molecular beam epitaxy (MBE) has been and remains the premier growth technique to explore new concepts in heterostructure physics and applications. We review the various effects which have been put to use in new physical phenomena or novel electronic and optoelectronic devices. The progress in the last few years both in basic research (electronic properties of the two-dimensional electron or hole gas, coherent light-matter coupling in microcavities,...), in materials choices (wide or small band gap III-V’s and II-VI’s, strained materials pairs,...), in structured growth (either lithography-aided or self-organized), in device concepts (electron "mirrors," structures with controlled spontaneous emission,...) evidences that the rate of appearance of novel MBE applications is undiminished, to the contrary.