Experimental studies will be described aimed at providing a scientific foundation for the use of single-walled carbon nanotubes (SWNTs) in biomedical applications. SWNTs have been found to be a unique class of nanoscale near-infrared (NIR) fluorescence contrast agents that also exhibit novel therapeutic capabilities. In our first study, we found that cultured macrophage cells readily engulf individual nanotubes. The rate of cellular uptake of SWNTs was measured by monitoring their characteristic NIR emissions. Furthermore, we also found that the NIR emissions of individual SWNTs are persistent in both the extracellular and intracellular environment of macrophage cells. Next, we extended our study from simple in vitro systems to the more complex in vivo mammalian animal model. By quantitatively tracking individual SWNTs, we have determined the rabbit's pharmacokinetic SWNT profile without the aid of additional fluorophores or radiolabels. As a final therapeutic application, we have developed SWNTs as novel pharmaceutical agents that efficiently carry siRNA molecules into cancer cells in order to induce targeted apoptosis of specific tumors.