Amyloids are fibrous protein aggregates that arise via polymerization of proteins with their concurrent conformational rearrangement and the formation of a specific cross-β structure. Amyloids are of particular interest as a cause of a vast group of human and animal diseases called amyloidoses. Some of these diseases are caused by prions, a specific type of amyloids, and are transmissible. Apart from mammals, prion amyloids are described in lower eukaryotes, where they act as nonchromosomal genetic determinants. Although amyloids are usually associated with pathologies in humans and animals, the increasing number of findings suggests that the acquisition of an amyloid or prion form by a protein is of biological significance in some cases. The review summarizes the data on the biological significance of prion and nonprion amyloids in a wide range of species from bacteria to mammals.