The challenges of quantifying and characterizing recent broad-scale changes in forest growth are substantial and exciting. The implications of these declines to the future growth and stability of forests in both Europe and the United States are significant. While the role of anthropogenic pollutants in initiating or exacerbating observed changes in growth and mortality is not clearly established, the possible implications of erroneous decisions with respect to pollution abatement are enormous and call for concerted, imaginative, and multidisciplinary research to provide much needed answers in the shortest possible time frame. Proof of cause and effect in complex natural ecosystems will not be absolute; however, diverse approaches can lead cumulatively to strong inferential evidence that substantially reduces the uncertainities of such decisions.