In this article, the syntheses and optical properties of core/shell quantum dot (CdSe/ZnS) and their applications are reviewed. Nevertheless, the main focus is to provide an overview on biological applications of quantum dots that contain imaging, targeting, and sensing. We discuss the different synthetic methods, optical properties (photoluminescence intensity, absorption, and fluorescence spectra), and their dependence on shape, size, and inner structure of quantum dots. Also, the different mechanisms of quantum dots bio-targeting (passive and active mechanisms) are discussed. The impact of quantum dots in bioimaging is reviewed regarding its photoluminescence intensity, absorption and emission spectrum, and photo-stability on high-quality and sensitivity imaging. Further, the difference between near infrared and visible emission quantum dots in deep tissue imaging will be reviewed and some of done works are considered and compared with each other. And finally, the biosensing potential/application of quantum dots in medical diagnosis is going to be highlighted.