Quantum dots (QDs) are luminescent semiconductor nanocrystals with unique chemical and physical properties due to their size and highly compact structure. QDs were first proposed for use in luminescent concentrators to replace organic dye molecules. In this book, the interest is in taking advantage of the emission properties of QDS, which can be tuned by their size, resulting from quantum confinement. In addition, the book discusses the potential of QDs as contrast and therapeutic agents in the field of medicine.