Semiconductor quantum dots are tiny light-emitting nanocrystals (2-10 nm) that have captivated researchers in the biomedical field in the last decade. Compared to organic dyes and fluorescent proteins, quantum dots (QDs) have unique optical properties such as tunable emission spectra, improved brightness, superior photostability, and simultaneous excitation of multiple fluorescence colors. Since the first successful reports on biological use of QDs a decade ago, QDs and their bioconjugates have been successfully applied in various imaging applications including fixed cell labeling, imaging of live cell dynamics, in situ tissue profiling, fluorescence detection, sensing and in vivo animal imaging. In this review, we will cover the optical properties of QDs, the biofunctionization strategies, their in vitro diagnostic applications and in vivo imaging applications. In addition, we will discuss the making of a new class of QDs--the self-illuminating QDs and their in vivo imaging and sensing applications. We will conclude with the issues and perspectives on QDs as in vivo imaging probes.