Modern biomedical imaging technologies have led to significant advances in diagnosis and therapy. Because most disease processes occur at the molecular and cellular levels, researchers continue to face challenges in viewing and understanding these processes precisely and in real time. The ideal imaging resolution would be in nanometers, because most biological processes take place on this length scale. Therefore, the functionalization of nanoparticles (NPs) and their use in therapeutic and diagnostic applications are of great interest. Molecular and cellular imaging agents made from inorganic NPs have been developed to probe such biological events noninvasively. The conjugation of tiny NPs with specific biomolecules allows researchers to target the desired location, reduce overall toxicity, and boost the efficiency of the imaging probes. In this Account, we review recent research on the functionalization of NPs for bioimaging applications. Several types of NPs have been employed for bioimaging applications, including metal (Au, Ag), metal oxide (Fe(3)O(4)), and semiconductor nanocrystals (e.g. quantum dots (QDs) and magnetic quantum dots (MQDs)). The preparation of NPs for bioimaging applications can include a variety of steps: synthesis, coating, surface functionalization, and bioconjugation. The most common strategies of engineering NP surfaces involve physical adsorption or chemisorption of the desired ligands onto the surface. Chemisorption or covalent linkages are preferred, and the coated NPs should possess high colloidal stability, biocompatibility, water solubility, and functional groups for further bioconjugation. Many of the functionalization techniques that have been reported in the literature suffer from limitations such as complex synthesis steps, poor biocompatibility, low stability, and hydrophobic products. Coating strategies based on chemisorption and ligand exchange often provide a better way to tailor the surface properties of NPs. After conjugation with the appropriate targeting ligands, antibodies, or proteins, the NPs may exhibit highly selective binding, making them useful for fluorescence imaging, magnetic resonance imaging (MRI), positron emission tomography (PET) imaging, and multimodal imaging.