The genetic code of viruses is typically protected by a spherical protein container. This protein capsid is assembled from hundreds of individual proteins in a spontaneous self-assembly process. We currently lack a detailed structural model for how self-assembly proceeds from the isolated subunits into the final protein capsid and how RNA/DNA gets incorporated into the capsids. In this proposal, we will study the capsid assembly process of hepatitis B virus using time resolved small angle neutron Scattering. Assembly of capsid will be triggered by mixing of capsid building blocks and RNA and the reaction will be followed over time. Using solvent contrast matching the structural transitions in protein and RNA over time can be followed separately and this will give a unique insight into the capsid assembly pathway.