Forest decline is a major environmental issue today. However, there are numerous examples of past declines in the condition of individual species within forests or of the entire forests themselves. Many of these declines are natural, being brought about by a variety of factors, including stand dynamics, pests and diseases. The emphasis that has been placed on air pollution in recent declines does not appear to be justified, although air pollution has undoubtedly brought about the decline of forests at some locations. Much of the literature exaggerates the extent of the decline in forest health in Europe. The overestimation of its extent has occurred because of the assessment techniques that are used: these are unable to distinguish trees that have recently declined from those that have always been in poor condition. In addition, the techniques that are most frequently used are nonspecific and no cause can be readily attributed to the defoliation that has been observed.