The Hubbard Brook Experimental Forest was established by the U.S. Forest Service in 1955 as a major center for hydrologic research in the Northeast. The Hubbard Brook Ecosystem Study originated 8 years later with the idea of using the small watershed approach to study element flux and cycling and the response of forest ecosystems to disturbance. Since that time, the research program at Hubbard Brook has expanded to include various physical, chemical and biological measurements collected by researchers from a number of cooperating institutions. Collaborative, long-term data are the keystone of the Hubbard Brook Ecosystem Study and have provided invaluable insight into how ecosystems respond to disturbances such as air pollution, climate change, forest disturbance, and forest management practices. This report highlights long- term ecological trends at Hubbard Brook, provides explanations for some of the trends, and lists references from the scientific literature for further reading.