Cationic liposome-DNA complexes, also called "lipoplexes", constitute a potentially viable alternative to viral vectors for the delivery of therapeutic genes. Here we review the mechanisms of lipoplex-mediated gene delivery, the barriers to efficient gene expression, and novel cationic lipids used for transfection. We also describe methods for enhancing gene transfer via the use of proteins, including transferrin, albumin and asialofetuin, and synthetic peptides, including GALA and nuclear localization signal peptides. We underscore the importance of understanding the mechanisms of cytoplasmic and nuclear entry of DNA and its dissociation from lipoplexes. We emphasize that the in vitro transfection activity of new lipoplex constructs should be tested in the presence of high serum concentrations to emulate in vivo conditions.