Beech bark disease is considered to be the most serious disease affecting British beech, although its severity varies geographically and temporally. Early records indicate that the disease was first observed at least 150 years ago, but must certainly have been affecting trees from a much earlier date. The insect and fungus both occur over much of the European range of Fagus sylvatica and the disease occurs in many forests within this area. The disease has also occurred since the early years of this century on American beech, Fagus grandifolia, in north-eastern USA and south-eastern Canada following the accidental introduction of C. fagisuga to Nova Scotia around 1890 and its subsequent spread to areas further west and south-west. The disease can be most simply described as the result of sequential attack by insect and fungus. This Bulletin considers this model first and then discusses the more complex aspects of disease development.