The entry of enveloped viruses into cells requires the binding of the virus to one or more cellular receptor and the fusion of the envelope with a cellular membrane. A large number of enveloped virus families, including single-stranded RNA viruses such as influenza viruses, alphaviruses, rhabdoviruses, and flaviviruses are internalized into cells by receptor-mediated endocytosis. After exposure to acid pH in endosomal compartments, a viral glycoprotein with the potential to mediate fusion undergoes a conformational change with the result that the viral envelope fuses with the endosomal membrane. Other enveloped viruses, including paramyxoviruses, coronaviruses, and retroviruses, including human immunodeficiency virus, have glycoproteins that mediate fusion at neutral pH (reviewed in White, 1990). While these viruses can cause fusion at the plasma membrane. it remains to be determined conclusively if successful entry leading to viral replication occurs via entry at the plasma membrane or whether successful entry depends on the virion being internalized and with pH-independent fusion occurring in endosomes. Regardless of the route of entry the most important consequence of viral membrane—cell membrane fusion is that the viral genome is released into the cytoplasm. While paramyxoviruses are known to fuse at neutral pH, there is considerable uncertainty about the roles played by each of the two viral glycoproteins and how the fusion process is triggered. The focus of this review concerns paramyxovirus-mediated fusion but first the properties of some better-character-ized fusion proteins of enveloped RNA viruses are considered because important paradigms and principles may be learned from them which bear on the roles of the paramyxovirus glycoproteins in the fusion process.