Many instances of forest decline, both natural and anthropogenic, can be attributed to combinations of stresses. In most cases, these combinations have proved to be much more complex than initially thought. In explaining the decline of trees or forests at any particular locality, it is important to consider all the stresses that may be operating. A useful framework is to divide stresses into predisposing, triggering and accelerating types, but these may not always be applicable. Generalisations about the role of pollution in forest decline cannot be made as there are many different types of decline and the importance of specific forms of pollution varies in each.