The methods detailed here measure the membrane fusion activity of viral fusion proteins and are optimized for the respiratory syncytial virus (RSV). RSV utilizes a class I fusion protein (F) to facilitate fusion between a viral and cellular membrane to allow viral entry into host cells. This process is carried out with the assistance of the glycoprotein (G), which attaches to the host-cell surface, allowing F to interact with the target cell. The F protein then undergoes conformational changes, which drive membrane fusion, thus creating a path for viral entry. The fusogenic activity of viral fusion proteins can be quantified by observing and measuring syncytia formation via wide-field microscopy. Syncytia are multinucleated cell masses formed by cell–cell membrane fusion, which in this case are driven by viral fusion proteins expressed on the host-cell surface.