Gene transfer could represent an important advance in the treatment of both genetic and acquired diseases. Cationic lipid-mediated gene transfer have advantages over viral gene transfer because they are non-immunogenic, they are easy to produce and they are not oncogenic. The major limitation of cationic lipid-mediated gene transfer is its inefficiency. The development of new cationic lipids, and the combination of a cationic lipid with a neutral lipid has conferred improved efficiency to cationic lipid-mediated gene transfer. The mechanism of gene transfer involves entry into the cell via endocytosis, escape from the endocytic compartment, uncomplexing the DNA from the lipids, and transport of DNA to the nucleus. In vivo gene therapy trials for cancer and cystic fibrosis, have shown promising results both in safety and efficacy.