Ash tree is an important symbol of the urban green space in the world, which is also used in the construction of home and sport equipment. Ash dieback disease caused by Hymenoscyphus fraxineus is widespread in the most forests and green areas of the European countries. The disease was first observed in Poland and Lithuania in the early 1990s. The geographical spread of the pathogen has increased in the last two decades and so it is now is considered as a serious threat to the Ash trees. Initial infection is caused by ascospores released from apothecia formed on the previous year's leaves dropped. Disease management can be achieved by prevention and quarantine methods, sanitation, identification and cultivation of resistant cultivars and the use of chemical fungicides. The disease has not been reported from Iran so far, however, the possibility of entering the disease in the future is unclear. Therefore in this article we discuss the various aspects related to this disease including symptoms, pathologic biology, and management methods.