Forests are subject to stress from natural causes and human activities. Biologic markers (i.e., indicators) associated with specific air pollutant stress and damage in forests have been sought to improve discrimination among the many anthropogenic and natural causes of stress. The U.S. Environmental Protection Agency asked the National Research Council to arrange a workshop to bring together researchers in forest science and complementary disciplines to summarize the current state of knowledge and research on biologic markers and identify and evaluate promising techniques that might lead to the development of new markers. The group reviewed the current and potential uses of biological markers at a variety of organizational levels, from the subcellular to the ecosystem level. The report is in two parts: the committee's report; and the 32 papers produced by individual participants who attended the workshop. The committee's report provides a summary of the workshop, a review of criteria for the establishment of cause and effect in complex relationships a discussion of the use of biologic markers to identify stress and damage in trees and forests, and the committee's conclusions and recommendations for further development and application of biologic markers.