Research on ash wilt disease, a common disease of Fraxinus excelsior L. in young forest and landscape plantings in several parts of the Netherlands, is described. By means of a survey for pathogenic fungi in affected trees, inoculation and reisolation experiments it is demonstrated that the disease is caused by Verticillium dahliae Kleb. Host specificity and virulence of a V.dahliae isolate from ash are compared to those of isolates from elm, maple and potato. Disease incidence and progress, and recovery of infected trees are investigated through monitoring experiments in two permanent plots in seriously affected forest stands. Monitoring results are related to the results of an aerial survey for ash wilt disease in the province of Flevoland to assess the impact of the disease on ash forests. Furthermore, pathological xylem anatomy of infected ash trees is described and mechanisms of recovery are discussed. Finally, results are integrated and related to data on ash and on verticillium. wilts of other tree species presented in literature reviews in the starting chapters of the book.