Maedi-visna (MV) is a persistent lentivirus infection of sheep. It is often grouped together with caprine arthritis-encephalitis (CAE) of goats as the small ruminant lentiviruses (SRLVs). Maedi-visna is also known as ovine progressive pneumonia (OPP). Maedi-visna is an Icelandic name that describes two of the clinical syndromes recognised in MV virus (MVV)-infected sheep. ‘Maedi’ means ‘laboured breathing’ and describes the disease associated with a progressive interstitial pneumonitis, and ‘visna’ means ‘shrinkage’ or ‘wasting’, the signs associated with a paralysing meningoencephalitis. Progressive lung disease is the primary finding with MVV infection. Phylogenetic analyses comparing nucleotide sequences of MV virus (MVV) and CAE virus (CAEV) have demonstrated that these are closely related lentiviruses. One source of CAEV and MVV transmission is colostrum and milk. The source of horizontal transmission in the absence of lactation remains unknown; however, faeces and lung fluids are known to harbour infectious virus. Clinical and subclinical MV and CAE are associated with progressive, mononuclear cell inflammatory lesions in the lungs, joints, udder and central nervous system. Indurative mastitis is common in both host species, and its economic significance may be underestimated. Laboured breathing associated with emaciation caused by progressive pneumonitis is the predominant feature in clinically affected sheep, whereas polyarthritis is the main clinical sign in goats. However, most lentivirus-infected sheep and goats are largely asymptomatic, but remain persistent carriers of virus and are capable of transmitting infection via colostrum or milk and respiratory secretions. There are no vaccines available. There is no evidence that humans are susceptible to any SRLVs.