Prions are self-perpetuating protein isoforms that are responsible for infectious diseases in mammals and for heritable traits in fungi. Most known prion proteins form amyloids--self-seeded fiber-like aggregates. Prion propagation ('replication') could be described as a sequence of repetitive cycles of aggregate 'shearing' into smaller seeds followed by the growth of these seeds into full-size polymers. Recent work shows that the ability to form aggregates and to propagate them is controlled by distinct regions of the composite prion domains (PrDs).