While immortalized cell lines traditionally have served as the common targets of transfection, primary cells are often preferred as they more closely resemble the in-vivo nature of tissues. However with the variability of primary cell types, there are challenges for efficient delivery of DNA or RNA. A given cell type may be amenable to a certain method but not another- and there a variety to choose from, such as by viral vector, polymer, liposomal and non-liposomal lipids, electroporation, and even magnetic particles. Fortunately, a number of commercial reagents have been demonstrated to successfully transfect a range of primary cells such as neurons, HUVECs, melanoma cells, among other difficult-to-transfect cells.