Self-assembled synthetic gene delivery systems represent the bottom-up approach to gene delivery and gene silencing, in which scientists are designing novel cationic and procationic amphiphiles that can pack, transport, and deliver nucleic acids to various targets in the body in a controlled manner. These supramolecular assemblies are safer than viruses, but they are lagging behind them in efficiency. We are presenting recent progress that has narrowed this gap through better understanding the delivery barriers and incorporation of this knowledge in the design of novel synthetic amphiphiles, formulations, and revolutionary screening and optimization processes. Structure-properties and structure-activity relationships were drawn within each amphiphile class, presenting the cellular and animal models used to generate them. We are also revealing pertinent in vitro/in vivo correlations that emphasize promising amphiphiles and successful formulation optimization efforts for efficient in vivo nucleic acid delivery, together with main organ targets and diseases treatable with these revolutionary technologies.