The capacity of tiny, noncoding RNA molecules (including small, interfering RNA molecules and micro-RNA molecules [miRNAs]) to control gene expression in a very specific and efficient manner has opened new avenues in biomedical research. RNA interference (RNAi) is now an important tool able to specifically inhibit the expression of almost any gene. The understanding of the molecular determinants of endocrine diseases has benefited a great deal from the new opportunities offered by the use of RNAi. Because RNAi is able to specifically inhibit the expression of particular genes it has great therapeutic potential, and the first clinical trials have already started. The delivery of RNAi in vivo, however, requires different methods to those used in vitro. RNAi uses several components of a cellular pathway devoted to the production of miRNAs, a class of naturally occurring small, noncoding RNA molecules that function as translational repressors. There is growing evidence that miRNAs play key regulatory roles in several cellular processes, such as proliferation, differentiation and apoptosis, and recent publications have demonstrated that alterations in miRNA function might be involved in endocrine diseases, including diabetes mellitus, and in endocrine cancer.