​This book analyzes and evaluates the growing field of light-emitting nanoprobes as contrast agents for in vivo imaging and sensing. It is a comprehensive resource that critically analyzes the state of the art in an interdisciplinary manner, with a special focus on the shift of emission wavelengths into the near-infrared (NIR) spectral region (ranging from 0.7 to 2 microns), which has greatly contributed to the latest advances in biomedical imaging and sensing. This book discusses merits of different contrast agents at nanoscale, and how their unique chemical and structural properties lead to the emission and interaction of light within the NIR window. Both the NIR-emitting materials and various surface modification strategies governing their interactions with the biological system at the “nano” level are discussed. Furthermore, different experimental techniques and protocols for NIR-light-based in vivo imaging and sensing are addressed to shed light on further understanding of the advantages and limitations of each category of these nanoprobes.