One of the fastest moving fields in nanotechnology is the application of quantum dots (QDs) in biology. These light-emitting materials are a new class of fluorescent labels in biomedical field. The superior properties such as resistance to photobleaching, a broad range of excitation and size-tunable light emission facilitate the applications of QDs in nanodiagnostics, imaging and targeted delivery. In addition, new synthesis approaches and modification strategies with several biodegradable polymers and biomolecules bring in water solubility features to QDs, which lead the way to the applications including cellular labelling, deep-tissue imaging and assay labelling. Here we discuss the synthesis, surface modification strategies and in vivo /in vitro biological applications of QDs.