Ash dieback is a disease that causes progressive death of branches and tree mortality in white ash and to a lesser extent in green ash (Fraxinus americana L., F. pennsylvanica March.). It affects all-aged trees in woodland, hedgerow, streetside, and home sites. Above-average tree mortality occurred in several northeastern states in the late 1950's and early 1960's. Recent surveys in New York show that the disease generally has become static, except in the southeastern Hudson Valley region. Here, for unknown reasons, ash dieback continues to increase in severity. Water stress in the trees and invasion of the bark by canker fungi are the primary causal factors. Air pollution, leaf spotting fungi, viruses, and mycoplasma-like organisms are additional stresses which may be involved in the etiology of ash dieback. Control recommendations are based on the maintenance of high tree vigor.