Quantum dots are tiny light-emitting particles on the nanometer scale. They are emerging as a new class of biological labels with properties and applications that are not available with traditional organic dyes and fluorescent proteins. Their novel properties such as improved brightness, resistance against photobleaching, and multicolor light emission, have opened new possibilities for ultrasensitive chemical analysis and cellular imaging. In this Research Highlight article , we discuss the unique optical properties of semiconductor quantum dots, surface chemistry and bioconjugation, current applications in bioanalytical chemistry and cell biology, and future research directions.