Theoretical analysis of the effect of reduction in dimensionality on the density of states for semiconductor material results in an idealized array of electrically isolated quantum-dots (QDs) with discrete states. Following consideration of the self-assembled growth process, this is replaced with a more accurate picture of a coupled QD-wetting layer electronic system. The factors that influence the gain that can be achieved from direct current injection of self-assembled QD material and the recombination pathways that influence the behavior of QD lasers are then discussed. In particular, p-doping, multilayer QDs, and tunnel-injection QD lasers (TI-QD) are introduced as methods to maximize performance of QD lasers. The properties that lead to particular benefits in using QDs as the active layer of a semiconductor laser are described. The chapter concludes with a discussion of QDs employed in high-power lasers, mode-locked lasers, semiconductor optical amplifiers, single-photon sources, and semiconductor integrated optoelectronics – five important application areas that take advantage of different elements of the properties of QD ensembles.