Biomolecular condensates formed by phase separation are membraneless compartments in the cytoplasm and nucleoplasm of cells, which have major roles in cellular organization and physiology. RNP granules are a specific type of condensate that assemble from RNA-binding proteins and RNA. In this talk, I will discuss how the concept of biomolecular condensates has expanded our view of RNP granules and their link to disease, aging and the cellular stress response. I will introduce in vitro reconstitution systems based on the concept of phase separation that now allow us to reconstruct RNP granules in the test tube. Using these reconstitution systems as well as cell biological and genetic approaches, we have gained important insights into the molecular rules of RNP granule assembly, such as the driving forces and amino acids that govern condensation, the conformational changes underlying assembly and molecular mechanisms of condensate regulation and control. I will further discuss how the concept of phase separation has allowed us to dissect the functions of RNP granules, and I will demonstrate how condensate formation can be used by cells to sense and respond to changes in the environment and regulate fundamental cellular processes such as protein synthesis.