Forest ecosystem health depends on stable forest composition and structure and on sustainable ecosystem processes. Forest disturbances that push an ecosystem beyond the range of conditions considered normal can upset the balance among processes, exacerbate forest health problems, and increase mortality beyond historical norms. Sometimes forest ecosystems respond to disturbances by returning to the normal range of conditions. At other times, however, the ecosystem is so altered that it follows a new trajectory—occasionally without historical precedent or known capability for resiliency—producing uncharacteristic changes in forest health and associated processes that may threaten the human, plant, or animal populations that depend on forests. The following sections describe forest health in northern landscapes using indicators based on overall mortality trends and on potential impacts of specific insects and diseases.