Delivery is a key issue in development of clinically relevant RNAi therapeutics. Polymeric nanoparticles formed by self-assembly of polycations with siRNA can be used for extracellular delivery, cellular uptake and intracellular trafficking as a strategy to improve the therapeutic potential of siRNA. This chapter describes a chitosan-based nanoparticle system for in vitro and in vivo transfection of siRNA into cells. The method exploits the mucoadhesive and mucopermeable properties of this cationic polysaccharide to deliver siRNA across mucosal epithelium and provides a platform for targeting human diseases with RNAi therapeutics.