Nanotechnology enables unique approaches to the diagnosis and treatment of cancer. Over the last two decades, a large number of nanoparticle-based cancer diagnostics and therapeutics have been developed. These include superparamagnetic iron oxide nanoparticles, gold nanoparticles, liposomes and polymeric nanoparticles. While some of the liposomal and polymer-based therapeutic nanoparticles have advanced to clinical applications, a greater number of remaining nanoparticles platforms are currently in the preclinical stages of development. They have relatively incomplete toxicity profiles at this time. In this chapter, we will review the available preclinical and clinical toxicity data of a variety of nanoparticle platforms.