Semiconductor quantum dots (QDs) are fluorescent nanocrystals normally smaller than 20 nm in diameter. Typically, they are composed of an inorganic core, made up of a few hundred to a few thousand atoms, surrounded by an organic outer layer of surfactant molecules. Bulk semiconductors are characterized by a composition-dependent bandgap which is the minimum energy required to excite an electron from the valence band into the conduction band. The most common QDs are binary semiconductor compounds. Depending on chemical components, they can be classified as II–VI, III–V, and IV–VI semiconductors. The synthesis routes for QDs have been well developed, beginning in the 1980s. Synthesis methods of colloidal QDs can be divided into two categories: wet-chemical synthesis and gas- (or vapor-) phase deposition. Synthesis has focused on producing QDs with better size and shape control and higher optical quality. Other factors, such as yield and green chemistry, have also been taken into consideration.