The development of safe, efficient and controllable gene-delivery vectors has become a bottleneck to human gene therapy. Synthetic polymeric vectors, although safer than viral carriers, generally do not possess the required efficacy, apparently due to a lack of functionality to overcome at least one of many intracellular gene-delivery obstacles. Currently, the exact mechanisms of how these polymeric vectors navigate each intracellular obstacle ("slit"), as well as their particular physical/chemical properties that contribute to efficient intracellular trafficking remain largely unknown, making it rather difficult to further improve the efficacy of non-viral polymeric vectors in vitro and in vivo. In this review, we first give a brief overview of synthetic polymeric vectors that have been designed and developed for gene delivery and highlight some promising candidates for clinical applications. Our main focus is on discussing the intracellular trafficking mechanisms of the DNA-polymer complexes ("polyplexes"), with less effort on the DNA-polymer complexation in the extracellular space as well as the in vivo systemic administration of genes in animal models and human clinical trials. In particular, we identified and discussed four critical, but often over-looked issues for successful DNA-polymer intracellular trafficking, especially our recent confirmation that it is free cationic polymer chains in the solution mixture of DNA and polymer that actually promote gene transfection and the polycationic chains within the polyplexes mainly play a protective role. Instead of the previously proposed and widely used escape model from late endolysosomes, the current hypothesis is that free polycationic chains with a sufficient length (∼20 nm) can block the initial endocytic-vesicle-to-endolysosome pathway.