Newcastle disease virus (NDV), a member of the Avulavirus genus in the Paramyxoviridae family, has a ribonucleic acid (RNA) genome that is negative sense, nonsegmented and single-stranded. The genome codes for six structural proteins: nucleocapsid, phosphoprotein (P), matrix, fusion, hemagglutinin–neuraminidase and the RNA-directed RNA polymerase, in addition to the nonstructural V protein that is produced by a frame shift in the P coding region. Virulent forms of NDV (vNDV) contain multiple basic amino acids in the fusion cleavage site along with a phenylalanine (F) at position 117, are found worldwide and are endemic in some countries. Infections of poultry species with vNDV lead to trade restrictions. Although all NDV strains are of one serotype, their genomes evolve over time, becoming more diverse. Strains of NDV have been used as viral vectors to formulate vaccines for other infectious diseases and experimentally to treat human cancers. The virus causes conjunctivitis in humans. Key Concepts: Virulent NDV (vNDV) is the aetiological agent of one of the most prevalent and virulent avian diseases in the world, and causes millions of birds to die, and significantly affects the poultry industries of many countries in Africa, Asia and South America. Lentogenic NDV (loNDV) is widely distributed in wild birds where it does not cause significant disease or clinical signs. Isolates of NDV are genetically diverse with at least two classes of viruses, each divided into 10 genotypes. Depending on the virulence of the NDV isolate and the immune status of the host, Newcastle disease produces a large range of clinical signs. The fusion protein cleavage site is a key determinant of virulence and a key element utilised for modern diagnostics and classification. Because all NDV isolates are of a single serotype vaccines prepared with any NDV isolate should protect against disease from any other NDV infection. Neutralising antibodies to the Fusion (F) and hemagglutinin–neuraminidase (HN) proteins of NDV are known to be important in preventing morbidity and morality from Newcastle disease. Even vaccinated birds with high antibody titre to NDV will become infected when exposed to virulent NDV and these vaccinated birds will shed virulent NDV in oral secretion and fecal matter. The genome of NDV has been used as a vaccine vector to carry foreign genes of other organisms for use in both animals and humans. Keywords: Avian paramyxovirus-1; APMV1; negative-strand RNA virus; avian diseases; NDV