Arenaviruses are the collective name for viruses, which belong to the family Arenaviridae. They replicate in the cytoplasm of cells, and were named after the sandy (Latin, arenosus) appearance of the ribosomes often seen in thin sections of virions under electron microscope. Several arenaviruses, such as Lassa virus in West Africa, and Junin, Guanarito, Sabia, Machupo, and Chapare viruses in South America, cause sever viral hemorrhagic fevers (VHF) in humans and represent a serious public health problem. These viruses are categorized as category 1 pathogens thus should be handles in a BSL4 laboratory. Recently, Lujo virus was isolated as a newly discovered novel arenavirus associated with a VHF outbreak in southern Africa in 2008. Although, we have no VHF patients caused by arenaviruses in Japan, except for a single imported Lassa fever case in 1987, it is possible that VHF patients occur as imported cases as for other VHF in the future. Therefore, it is necessary to develop the diagnostics and therapeutics in consideration of patient's severe symptoms and high mortality even in the disease-free countries. In this review, we will broadly discuss the current knowledge from the basic researches to diagnostics and vaccine developments for arenavirus diseases.