Mesenchymal stem cells (MSCs) are considered as the adult stem cells with the greatest therapeutic potential, due to characteristics such as plasticity, intrinsic tropism towards lesions, paracrine activity, and immunosuppressive activity. This potential may be optimised by transforming them with genes which will improve their therapeutic ability or are therapeutic by themselves. This review presents a summary of the main types of viral or plasmid vectors used to transform therapeutic MSCs and their use in different pathologies. Although this strategy holds great promise, results are still heterogeneous, showing that more studies are needed to optimise gene transfer methods and models.