A key element of any nanomaterial toxicity screening strategy is a detailed and comprehensive physicochemical characterization of the test material being studied. This is a critical factor for correlating the nanoparticle surface characteristics with any measured biological/toxicological responses, as well as to provide an adequate reference point for comparing toxicity results with the hazard-based findings of other investigators. Moreover, when hazard or risk-based evaluations are made on a particular nanomaterial (based on a variety of studies), it is important to ensure that the nanoparticle-types are identical or very similar in composition. This can only be accomplished if rigorous characterization is conducted. In the absence of an adequate assessment of the physical characteristics, it is easy to draw general conclusions on nanoparticle-types which may have similar chemical compositions but, in fact, have different sizes, shapes, crystal structures, surface coatings, and surface reactivity characteristics. The determination of nanomaterial physicochemical properties is vitally important to nanomedicinal applications in that the fate, accumulation, and transport of nanomaterials through the body over time may be predicted based on specific surface characteristics. Copyright © 2009 John Wiley & Sons, Inc.