Nanoparticles possess enormous potential as diagnostic imaging agents and hold promise for the development of multimodality agents with both imaging and therapeutic capabilities.Yet,some of the most promising nanoparticles currently under investigation demonstrate prolonged tissue retention and contain heavy metals.This presents serious concerns for toxicity and additionally,prolonged particle retention may interfere with diagnostic imaging and testing modalities.The creation of nanoparticles with optimal clearance characteristics will minimize toxicity risks by reducing the duration of exposure to these agents.Given that many types of nanoparticles possess easily modifiable surface chemistry,if nanoparticle characteristics associated with optimal clearance from the body were well established,it would be feasible to design and create agents with more favorable clearance properties.This paper presents a thorough discussion of the physiologic aspects of nanoparticle clearance as well as an overview of current research investigating the clearance of the distinct types of nanoparticles,including dendrimers,quantum dots,liposomes and carbon,gold,and silica-based nanoparticles.