The vaccinia virus-based expression of viral envelope glycoprotein genes—derived from enveloped viruses that infect their respective host cells through a pH-independent mechanism of membrane fusion—has been a powerful tool in helping to characterize these important attachment and fusion proteins. The cellular expression of these viral envelope glycoproteins has allowed for the measurement of membrane fusion events using cell-cell fusion or syncytia formation. This method has been enhanced by the addition of a reporter-gene system to the vaccinia virus-based cell-cell fusion assay. This improvement has provided a high-throughput and quantitative aspect to this assay, which can serve as a surrogate for virus entry and is therefore ideally suited in the characterization of numerous enveloped viruses, including biological safety level-4 (BSL-4) agents. This chapter will detail the methods of the vaccinia virus-based reporter-gene fusion assay and how it may be used to characterize the fusion mediated by the BSL-4-classified Hendra and Nipah viruses.