Lipid nanoparticle (LNP) technology is increasingly enabling RNA-based gene therapies that can potentially be used to treat most diseases. Further, these LNP RNA therapeutics can be designed and manufactured in a matter of weeks, allowing personalized medicines that can be produced in a time frame relevant to individuals suffering from terminal diseases. Here, we focus on the rational design principles that have successfully enabled LNP small interfering RNA (siRNA) formulations to silence pathogenic genes in the liver and LNP mRNA formulations to express therapeutic proteins for vaccines and gene therapies. These principles have evolved from over 50 years of research into the physical properties and functional roles of lipids in membranes as well as experience gained developing LNP systems for delivery of small molecule drugs. It is expected that these rational design principles will be successful in enabling most forms of gene therapies.