To understand and treat any disease with a genetic basis or predisposition, scientists and clinicians need effective ways of manipulating the levels of genes and gene products. Conventional methods for the genetic modification of many experimental organisms are technically demanding and time consuming. Just over 5 years ago, a new mechanism of gene-silencing, termed RNA interference (RNAi), was discovered. In addition to being a fascinating biological process, RNAi provides a revolutionary technology that has already changed the way biomedical research is done and that may even prove useful for genetic interventions in a clinical context. In this course, students learn how RNAi was discovered, how it works, and what its physiological relevance might be. How RNAi can be harnessed to modulate gene expression and perform genetic screens, both in cells and in various organisms is also covered. Finally, this course examines the first attempts to use RNAi for the treatment of models of human diseases in experimental animals.