The epidemiology of Rift Valley Fever has undergone extensive revision over the last twenty years since the epidemics that occurred in Egypt in 1977 and Mauritania in 1987. From 1931 when the disease was first described until the end of the 70s, Rift Valley Fever was considered to be a relatively benign zoonoses for man that developed in domestic animals (especially sheep) periodically following heavy rainy seasons. The presumptive reservoir for the virus in latent years was an unidentified animal living in the wilderness. However the outbreaks in Egypt and Mauritania and data obtained from ensuing research led to complete reassessment of this assumption. Current data suggest that environmental factors (especially ecological changes caused by development of hydroelectric resources) and low-grade transmission in domestic animals (especially rodents) could account for survival of the virus in zones uninhabited by wild animals and in Sahelian areas.