Book excerpt: This book presents an overview of the current understanding of the physics of zero-dimensional semiconductors. It concentrates mainly on quantum dots of wide-gap semiconductors, but touches also on zero-dimensional systems based on silicon and III-V materials. After providing the reader with a theoretical background, the author illustrates the specific properties of three-dimensionally confined semiconductors, such as the size dependence of energy states, optical transitions, and dephasing mechanisms with the results from numerous experiments in linear and nonlinear spectroscopy. Technological concepts of the growth concepts and the potential of this new class of semiconductor materials for electro-optic and nonlinear optical devices are also discussed.