RNAi interference (RNAi) is an almost standard method for the knockdown of any target gene of interest in vitro, exploring a naturally occurring catalytic mechanism. Beyond functional analyses, the downregulation of pathologically relevant genes which are aberrantly expressed in a given disease will offer novel therapeutic approaches, also with regard to otherwise 'undruggable' genes. RNAi is mediated by small interfering RNAs (siRNA), and thus siRNA delivery in vivo is of critical importance for its implementation. Due to the instability and physicochemical properties of siRNAs, the development of strategies and formulations for siRNA protection, cellular uptake, correct intracellular localization and endosomal release, in combination with favourable pharmacokinetic properties, preferential delivery to the target organ, high biocompatibility and absence of unwanted side effects is crucial for the success of RNAi-based therapeutics. Approaches include the encapsulation in lipids, the complex formation with a variety of liposomes or cationic polymers, the chemical conjugation of siRNAs for example to peptides, aptamers or antibodies as well as other formulations. This review discusses non-viral strategies, based on different siRNA formulations and various modes of administration, for the delivery of therapeutic siRNAs to induce RNAi in vivo. It gives a comprehensive overview including a detailed listing of in vivo studies which have successfully employed various strategies for analytical or therapeutic siRNA-mediated gene targeting in different animal models, and presents a more in-depth description of some promising approaches with a special emphasis on polymers.