**1. Introduction**

Quantum dots are small regions defined in the semiconductor materials with the same size of the distance in an electron-hole pair [1]. The physics of quantum dots has been a very active and fruitful research topic. Their unique optical, photochemical, semiconductor, and catalytic properties are due to the quantum confinement. To date, chemistry, physics, and materials science have provided methods for the production of quantum dots and allow tighter control of factors affecting, for example, particle growth and size, solubility and emission properties. This book deals with the electronic and optical properties of quantum dots as an artificially fabricated device. These dots have proven to be useful systems to study a wide range of physical phenomena. These characteristics provide the potential applications of quantum dots in photovoltaic and laser devices, thin-film transistors, light-emitting diodes, and luminescent labels in biology and medicine. Some of these applications are discussed in separate sections in this book.
