Quantum dots (QDs) are powerful tools for bioimaging and biosensing because of their superior optical properties, which outperform most organic fluorophores. They have been extensively used for both in vitro detection and in vivo imaging of disease-related biomarkers such as cell receptors, protease, and nucleic acids with high specificity and sensitivity. This chapter provides an overview of the design considerations and synthetic routes of QD-based probes, the main principles and mechanisms for QDs-based biosensing and bioimaging, and the use of QDs for simultaneous disease diagnostics and therapy.