For the purpose of introducing nucleic acids into cells, cationic polymers have been steadily improved as gene carriers. This has resulted in improved polymer-based gene transfer formulations, termed polyplexes, which efficiently transfect cell cultures and also have shown encouraging gene transfer potential in in vivo administration. Targeted delivery to liver, lung, tumor, or other tissues has been achieved in experimental animals by localized or systemic application. Therapeutic effect has been demonstrated, although efficiencies are still too low to justify clinical use. The limitations of first-generation polymeric carriers (modest activity and significant toxicity) have been addressed by developments of new biodegradable polycations, incorporation of targeting and intracellular transport functions, and polyplex formulations that avoid unspecific adverse interactions with the host. A key future step will be the development of polyplexes into artificial viruses, with virus-like entry functions presented by smart polymers and polymer conjugates. These polymers have to sense their biologic microenvironment, respond in a more dynamic manner to alterations in pH, ionic or redox environment, undergoing programmed structural changes compatible with the different gene delivery steps.