Segments of viral fusion proteins play an important role in viral fusion. They are defined by a number of criteria, including the sensitivity of this region of the viral fusion protein to loss of function as a consequence of mutation. In addition, small model peptides designed to mimic this segment of viral fusion proteins often have some membrane perturbing activity. The properties of viral fusion peptides are quite varied. Many are found at the amino terminus of viral fusion proteins. As isolated peptides, they have been found to form both alpha-helical as well as beta-structure. In addition, some viruses have internal fusion peptides. Just as there are several structural motifs for viral fusion peptides, there are also several mechanisms by which they accelerate the process of membrane fusion. These include the promotion of negative curvature, lowering the rupture tension of the lipid monolayer, acting as an anchor to join the fusion membranes, transmitting a force to the membrane or imparting energy to the system by other means. It is not likely that the fusion peptide can fulfill all of these diverse roles and future studies will elucidate which of these mechanisms is most important for the action of individual viral fusion peptides.