Key Points RNA interference (RNAi) is a fundamental pathway in eukaryotic cells by which sequence-specific small interfering RNA (siRNA) is able to silence genes through the destruction of complementary mRNA. RNAi is an important therapeutic tool that can be used to silence aberrant endogenous genes or to knockdown genes essential to the proliferation of infectious organisms. Delivery remains the central challenge to the therapeutic application of RNAi technology. Before siRNA can take effect in the cytoplasm of a target cell, it must be transported through the body to the target site without undergoing clearance or degradation. Currently, the most effective synthetic, non-viral delivery agents of siRNA are lipids, lipid-like materials and polymers. Various cationic agents including stable nucleic acid–lipid particles, lipidoids, cyclodextrin polymers and polyethyleneimine polymers have been used to achieve the successful systemic delivery of siRNA in mammals without inducing significant toxicity. Direct conjugation of delivery agents to siRNA can facilitate delivery. For example, cholesterol-modified siRNA enables targeting to the liver. RNAi therapeutics have progressed to the clinic, where studies are being conducted to determine siRNA efficacy in treating several diseases, including age-related macular degeneration and respiratory syncytial virus. Moving forward, it will be important to pay close attention to the potential nonspecific immunostimulatory effects of siRNA. Modifications to siRNA can be used to minimize stimulation of the immune system, and an increased emphasis must be placed on performing proper controls to ensure that therapeutic effects are sequence-specific.