After more than 20 years of intensive investigations, gene therapy has become one of the most promising strategies for treating genetic diseases. However, the lack of ideal delivery systems has limited the clinical realization of gene therapy's tremendous potential, especially for DNA-based gene therapy. Over the past decade, considerable advances have been made in the application of polymer-based DNA delivery systems for gene therapy, especially through multifunctional systems. The core concept behind multifunctional polymeric DNA delivery systems is to endow one single DNA carrier, via materials engineering and surface modification, with several active functions, e.g., good cargo DNA protection, excellent colloidal stability, high cellular uptake efficiency, efficient endo/lysosome escape, effective import into the nucleus, and DNA unpacking. Such specially developed vectors would be capable of overcoming multiple barriers to the successful delivery of DNA. In this review, we first provide a comprehensive overview of the interactions between the protein corona and DNA vectors, the mechanisms and challenges of nonviral DNA vectors, and important concepts in the design of DNA carriers identified via past reports on DNA delivery systems. Finally, we highlight and discuss recent advances in multifunctional polymeric DNA delivery systems based on "off-the-shelf" polycations including polyethylenimine (PEI), poly-l-lysine (PLL), and chitosan and offer perspectives on future developments.