The Long-Term Ecological Research (LTER) program of the National Science Foundation was formulated and initiated by the Division of Biotic Systems and Resources to support research that addresses ecological variabilities that may last or be detectable only over a period of 10 to 100 years. Eleven sites have been selected at which research in various ecological areas is being conducted by interdisciplinary groups of scientists with backgrounds in hydrology, biology, geology, forestry, geography, chemistry, and other disciplines. At all the sites, data collection and analyses are being performed on various biological processes that represent the total environment of these sites. Research on hydrological processes and their interactions with biology is a vital part of the long-term ecological research projects. This article summarizes various topical hydrological research and data collection projects that are being performed at these sites.