The development and application of semiconductor light-emitting and laser diodes has been a huge success during the last 30 years in key areas of modern technology like communications, recording, and printing. Still there is ample room for improvement through combination of the atomlike properties for zero-dimensionally localized carriers in quantum dots (QDs) with state-of-the-art semiconductor-laser technology. Low, temperature-insensitive threshold current; high gain; and differential gain have been predicted since the early 1980s.In the past two decades, the fabrication of QDs has been attempted using colloidal techniques (see the article by Nozik and Mićić in this issue), patterning, etching, and layer fluctuations (see the article by Gammon in this issue). However a break-through occurred recently through the employment of self-ordering mechanisms during epitaxy of lattice-mismatched materials (see the next section) for the creation of high-density arrays of QDs that exhibit excellent optical properties, particularly high quantum efficiency, up to room temperature. The zero-dimensional carrier confinement and subsequent atomlike electronic properties have a drastic impact on optical properties (see the section on Spectroscopy). Also intimately connected is the applicability of QDs as a novel gain medium in state-of-the-art laser diodes with superior properties (see the section on Lasers).