Nanometer-sized particles, such as semiconductor quantum dots and iron oxide nanocrystals, have novel optical, electronic, magnetic or structural properties that are not available from either molecules or bulk solids. When linked with tumor-targeting ligands, such as monoclonal antibodies, peptide fragments of tumor-specific proteins or small molecules, these nanoparticles can be used to target tumor antigens (biomarkers) and tumor vasculatures with high affinity and specificity. In the mesoscopic size range of 5-100 nm diameter, quantum dots and related nanoparticles have large surface areas and functional groups that can be linked to multiple diagnostic (e.g., optical, radioisotopic or magnetic) and therapeutic (e.g., anticancer) agents. In this review, recent advances in the development and applications of bioconjugated quantum dots and multifunctional nanoparticles for in vivo tumor imaging and targeting are discussed.