Gene therapy, currently, represents one of the new therapies that have emerged for the treatment of different diseases. Between the different approaches that can be chosen concerning this therapy, gene delivery with non-viral systems has focused the attention during the past decades, because of the reduced toxicity compared to the viral systems. Non-viral vectors are formed by a group of different molecules with a wide variety of sources and features. Among them, polymeric systems have been extensively studied due to the ability to form nanoparticles in the presence of nucleic acids, protect the included nucleic acid and more importantly, improve the entrance of the desired nucleic acid fragment into the cell and hopefully achieve a therapeutic effect. In this review, some of the most used polymeric systems are commented with the main characteristics that can influence the activity of each of them, such as the molecular weight, N/P ratio (positive charges of cationic polymer/negative charges of pDNA), stability or the molecular structure.