RNA interference (RNAi) has rapidly advanced from a laboratory observation into a major area of research within biology and medicine. RNAi is triggered by short interfering RNAs (siRNAs) of between 19 and 21 nucleotides in length, which induces the targeted cleavage of mRNA with sequences of homology to the siRNA. Because of its high degree of specificity and efficacy, the potential for RNAi-based therapeutics was recognized at an early stage. However, development of RNAi-based agents has been hindered because siRNAs are unstable in serum and delivery across the cell membrane is highly inefficient. Numerous methods have been developed to facilitate delivery of RNAi in animals and patients, each with their own set of advantages and disadvantages. This review discusses publications between 2005 to 2007 in the area of RNAi delivery, with a particular focus on in vivo application and clinical trials.