This book assesses the current state of knowledge about forest declines, particularly as they relate to air pollution. Many declines are shown to be unrelated to air pollution and, in other cases, the links between air pollution and the condition of the trees is extremely tenuous. Factors such as nutrient depletion due to past management practices and drought are shown to have had a major influence on the health of trees. In many cases, the evidence for a decline in tree condition is at best ambiguous, and much of the concern appears to have been generated by political and media interest rather than by a critical evaluation of the available data. A detailed examination of the comprehensive data collected in the British forest health monitoring programme over the last five years indicates the complexity of the process involved and illustrates the dangers of taking data from a single year at face value. Many of the indices used to assess trees are of questionable value when compared over time or space, as their subjective nature precludes any reliable comparisons. Studies of annual increment and wood quality indicate that the international market for timber is unlikely to be affected in either the long or the short term.