Single walled carbon nanotubes (SWNTs) are emerging as promising delivery vehicles for cancer diagnostics and chemotherapies due to their unique properties, including, remarkable cell membrane penetrability, high drug-carrying capacities, pH-dependent therapeutic unloading, prolonged circulating times and intrinsic fluorescent, photothermal, photoacoustic and Raman properties. In this leading opinion paper, we systemically discuss and evaluate the relationship of the biological safety of SWNTs with their physicochemical properties such as their length, purity, agglomeration state, concentration and surface functionalization. Other relevant issues, including the cellular uptake mechanism, biodistribution and metabolism of SWNTs are also reviewed. The design and preparation of SWNT-based drug delivery systems (DDSs) and their pharmacokinetic, cancer targeting and therapeutic properties both in vitro and in vivo are highlighted. Future opportunities and challenges of SWNT-based DDSs are also discussed. (C) 2011 Elsevier Ltd. All rights reserved.