Measles virus is the prototypic member of the Morbillivirus genus of the family Paramyxoviridae. The viral genomic RNA is single-stranded, nonsegmented, and of negative polarity and encodes six major structural proteins. The two viral transmembrane glycoproteins, the hemagglutinin and fusion proteins, are both required for virus-host cell membrane fusion, while attachment to host cells is mediated by the hemagglutinin. The human CD46 molecule has been identified as a cellular receptor for measles virus. Antibodies raised against either viral glycoprotein neutralize measles virus in vitro and protect against infection. Although measles virus remains a single serotype (monotypic), nucleotide sequence analyses have identified distinct lineages among recent wild type isolates. These genetic changes were manifested by detectable antigenic variation between vaccine and wild type viruses and at some point may influence strategies for control, elimination, and eventual eradication of measles virus.