Human Respiratory Syncytial Virus (RSV) is the main viral agent causing bronchiolitis in young children and a major pathogen of elderly and immunocompromised people, responsible for acute respiratory infections. Despite this burden, no vaccine or effective treatment against RSV is currently available. The RSV genome replication and transcription take place in cytoplasmic viral factories (VFs) where viral and cellular proteins involved in these activities concentrate. The genome, encapsidated by the nucleoprotein N into a helical nucleocapsid (NC), serves as template for the viral RNA synthesis by the polymerase L, its phosphoprotein cofactor P and the elongation factor M2-1. Together, the helical NC, P, L and M2-1 form the ribonucleoprotein (RNP) responsible for viral RNA synthesis. Although the structures of these proteins have been solved and the protein-protein interactions involved in the polymerase functioning characterised, the molecular architecture of the RNPs within VFs and the structural states of the NCs supporting the distinct activities of the polymerase remain unknown. Here, we propose to unravel the nanoscale organisation of the RSV VFs and the different functional states of the NCs by a parallel investigation of NCs in VFs and pseudo-VFs inside the cell, purified or reconstituted in vitro. This goal is challenging due to the inherent compositional heterogeneity, dynamics and environment of the VFs, and conformational flexibility of the NCs. We aim to achieve it by using an original multi-scale approach that relies on complementary state-of-the-art techniques such as advanced cellular imaging, cryo-CLEM, cryo-FIB/SEM, cryo-ET, cryo-EM, FAPS, and biochemical and biophysical characterisation. The project outcome will improve our understanding of the regulation of the RSV RNAs synthesis and contribute to innovative rational design of new antivirals.