An excited electronic state in a large molecule, or a semiconducting or insulating solid composed of a negatively charged electron and a positively charged hole bound together by their electrical attraction. Its creation through internal charge separation is most frequently caused by the absorption of light, and its demise is occasioned by the emission of light. The exciton is thus an important portal for the study of the properties of an insulator by optical means and for applications such as optoelectronics. Its external neutrality enables it to move through the system as an energy carrier, an important agent in energy transfer processes of special importance in biological functions such as photosynthesis. When the excited electron and hole are restricted to a plane, a wire, or a microscopic box in an artificially fabricated structure known, respectively, as a quantum well, quantum wire, or quantum dot, a new dimension is added to the ability to explore fundamental nonlinear and nonequilibrium physics and coherent quantum optics as well as quantum control of microscopic objects.