If forest health is to be approached scientifically, it must be defined and measured. Forest health is a condition of forest ecosystems that sustains their complexity while providing for human needs. We developed this broad definition becaue a wisely acceptable definition is lacking, and forest health is a focal point in discussions of how to sustain forest ecosystems in the United States. Steps for measuring forest health are: (1) select a representative set of indicators for a particular ecosystem; (2) establish baseline data, such as a historical range of variability; (3) develop standards against which to compare current conditions; and (4) establish a monitoring program to assess current conditions and modify baseline data as new trends develop.