Nanotechnology is an area of scientific research and technology development dealing with the structures and devices with length scales in the 1- to 100-nanometer range. Multifunctional nanoparticles that can target, diagnose, and treat diseases such as cancer are designed and developed. Numerous studies have shown that anticancer drugs encapsulated into nanoparticles can be delivered to target tumor cells and tissue. This would increase antitumor efficacy and reduce systemic side effects. Quantum dots are kind of nanoparticles with unique photochemical and photophysical properties. They are several orders of magnitude brighter than conventional fluorophores, and their emission spectra are very narrow and can be tuned by adjusting the size of the dots. Quantum dots are a new class of fluorescent labels with improved brightness and resistance against photobleaching. These properties could improve the sensitivity of biological detection and imaging by at least 10- to 100-fold. In this review, data of tumor targeting with conventional and long-circulating nanoparticles as well as applications of semiconductor quantum dots for in vivo imaging are discussed.