The advent of recent technologies such as gene expression microarrays and high-throughput sequencing methods has allowed for unveiling the molecular complexity of cancer. However, compared to the genomic discovery stage, the functional characterization of genes that have been found altered (by somatic mutations, rearrangements, or copy number variations) or differentially regulated at the expression level is still lagging behind. In the future, it is anticipated that efforts would be aimed at addressing the impact of such genes on several cancer traits, including tumor formation, dissemination, and response to therapies. These studies would likely have to rely on introducing the gene(s) of interest (in its wild-type or altered version) in cellular models. We describe here a number of techniques to introduce nucleic acids into eukaryotic cells, ranging from conventional plasmid transfection to lentiviral transduction and adeno-associated viral (AAV)-mediated DNA transfer.