The innate immune system represents the first line of defense developed by organisms to fight against infections. Its efficiency strictly depends on its ability to recognize pathogens as foreign, i.e. on its capacity to discriminate between self and non-self molecules. To do so, the cell uses specific receptors such as RIG-I, that is dedicated to the recognition of RNA viruses in the cytoplasm. How does RIG-I discriminate viral RNAs from cellular RNAs? What are the molecular features which are recognized by RIG-I and activate it? What are the strategies set up by viruses to avoid immune recognition? To answer these questions, an extensive study based on synthetic RNAs has been performed. These results were then applied to the model of Influenza virus. Altogether, while providing a functional validation of the structural data obtained by crystalizing this cytoplasmic sensor, this works allowed us to define a new model of RIG-I activation.