Avian pneumovirus (APV, Turkey rhinotracheitis virus) and Human metapneumovirus (hMPV) are pathogens of birds and humans, respectively, that are associated with upper respiratory tract infections. Based on their different genomic organization and low level of nucleotide (nt) and amino acid (aa) identity with paramyxoviruses in the genus Pneumovirus, APV and hMPV have been classified into a new genus referred to as Metapneumovirus. First isolated in 1970s, APV strains have since been isolated in Europe, Africa, middle east, and United States (US) and classified in four subgroups, APV/A, APV/B, APV/C, and APV/D based on nt and predicted aa sequence identity. Although it was first isolated in 2001, serological evidence indicates that hMPV may have been present in human population from as early as the 1950s. There is only one subgroup of hMPV so far, whose nt and aa sequence identity indicates that it is more closely related to APV/C than to APV/A, APV/B, or APV/D.