Gene therapy will change medicine by treating the diseases at their core levels revolutionizing the way to deliver functional proteins. The development of this technology relies in designing optimal systems for DNA transfer and expression (transfection), cationic lipids being a promising alternative. Being safer than viral vectors, they also allow the delivery of larger plasmids and can be easily GMP-manufactured and stored. The main problem associated with the use of these vectors is their transfection efficiency, which is still inferior to viral methods. In this paper we present an overview of the correlations between the chemical structure and biological activity for the principal classes of cationic lipids. Key issues in the design of this class of transfection agents are presented, as well as the future trends.