After a decade of effort, a variety of highly luminescent noble metal nanoparticles (NPs) with different sizes and surface chemistries have been developed for in vivo imaging. Among them, the near infrared (NIR)-emitting gold NPs (AuNPs) with small hydrodynamic diameters, well-designed surface chemistry, and high physiological stability are very promising candidates for in vivo tumor imaging. In this chapter, we summarize recent progress in the synthesis strategies and in vivo behavior of NIR-emitting AuNPs in terms of renal clearance, pharmacokinetics, and in vivo tumor imaging applications. Finally, we also discuss the future challenges and opportunities for them.