A quantum dot laser is a semiconductor laser that uses quantum dots as the active medium for stimulated emission of light. Due to quantum confinement of charge carriers in all three spatial directions, their energy spectrum in quantum dots is discrete and resembles that in atoms. Injection lasers based on semiconductor quantum dot heterostructures promise device characteristics superior to traditional semiconductor lasers based on quantum wells and even more so bulk active medium. Improvements in lasing threshold, relative intensity noise, linewidth enhancement factor and temperature-insensitivity have already been demonstrated in quantum dot lasers. The quantum dot active region may also be engineered to operate at different wavelengths by varying dot sizes and composition. This allows to fabricate quantum dot lasers operating at wavelengths beyond those achievable in quantum well lasers.