The most common approach to delivery in vitro involves the use of recombinant viruses as gene carriers due to their high transduction efficiencies, which can result in high levels of protein expression. However their use in vivo has been hampered by the fact that many of the viral proteins trigger strong immune responses and scaling up recombinant virus-based delivery systems remains challenging. Non-viral gene delivery systems, including cationic lipids, polymers, dendrimers, and peptides, show significantly reduced transfection efficiencies compared to the viral systems. Recently, a new avenue of research has focused on nanoparticles as delivery vehicles. Careful engineering of their surface properties with specific recognition elements such as antibodies has provided an ability to target specific cells . In fact, vectors based upon a variety of nanoscale carriers, including carbon nanotubes, iron oxide, silica, and gold nanoparticles have all demonstrated successful gene delivery. Gold nanoparticles are of particular interest as they are biologically inert, which by implication suggests that they should not be cytotoxic. They are easily synthesized and, as stated, are readily functionalized using established thiol chemistries, enabling the engineering of the surface with receptors. A complex picture has now emerged within the literature, where nanoparticle biocompatibility can be seen to be dependent upon dose, cell type and surface properties. For example, while no toxicity has been found in studies using gold nanoparticles in BHK21 and HepG2 cells, in others cell lines such as A549, the opposite is true. The fact that the surface of gold nanoparticles offers well established routes for functionality makes them appealing vehicles. Charged or hydrophobic motifs can be readily bound to the surface, and indeed can be combined with modification protocols that include targeting moieties such as antibodies and receptors. These too can be mixed with a payload such as double-stranded DNA or single-stranded DNA, which is then transfected into cells. In one example, Rosi et al. demonstrated the knockdown of genetically-encoded enhanced green fluorescent protein after transfection with ssDNAfunctionalized gold nanoparticles by measuring fluorescence of the residual target protein in the cell. In a more recent study, Kim et al. were able to demonstrate the knockdown of p53 protein in HeLa cells using ssDNA-functionalized gold nanoparticles. Nanoparticles have been shown to enter cells in one of two ways, either by passive uptake, possibly diffusion, or by endocytosis. In the latter case, the particles migrate through the cells via a variety of different vesicles to reach the nucleus. The different transfection reagents that have been developed to facilitate their uptake either by a passive or active mode, influence not only the efficiency of transfection, but also the localization of the payload after it enters the cell.