Filoviruses (Ebola and Marburg viruses) cause severe hemorrhagic fever in humans and nonhuman primates. No effective prophylaxis or treatment for filovirus diseases is yet commercially available. Recent studies have advanced our knowledge of filovirus protein functions and interaction between viral and host factors in the replication cycle. Current findings on the ecology of filoviruses (i.e., natural infection of nonprimate animals and discovery of a new member of filoviruses in Europe) have also provided new insights into the epidemiology of Ebola and Marburg hemorrhagic fever. This article reviews the fundamental aspects of filovirus biology and the latest topics on filovirus research.