Viral gene therapy has high efficacy, but is plagued by serious safety risks, production and manufacturing challenges, and other limitations including nucleic acid cargo capacity. To address these challenges, we have developed poly(beta-amino ester)s, easy-to-synthesize biodegradable polymers, that are able to deliver DNA to primary human umbilical vein endothelial cells (HUVECs) at levels comparable to adenovirus at a Multiplicity of Infection (MOI) between 100 and 500, and two orders of magnitude better than the commonly used non-viral polymeric vector, polyethylenimine (PEI). Interestingly, small structural changes to these polymers were found to have dramatic effects on gene delivery including tightening of the polymer/DNA binding constants, altered biophysical properties of the DNA-encapsulating nanoparticles, and enhanced subsequent protein expression. In vivo, these polymer modifications dramatically enhance DNA delivery to ovarian tumors. We also demonstrate a general method of coating these polymer/DNA nanoparticles for targeted delivery. At appropriate formulation conditions including near-neutral charge ratio, the coated nanoparticles enable effective ligand-specific gene delivery. We believe the development of this nanoparticle drug delivery system with virus-like efficacy, ligand-based specificity, biodegradability, and low cytotoxicity, could set a new benchmark in non-viral transfection capability.