The AmeriFlux network of eddy flux covariance towers was established to quantify variation in carbon dioxide and water vapor exchange between terrestrial ecosystems and the atmosphere, and to understand the underlying mechanisms responsible for observed fluxes and carbon pools. The network is primarily funded by the U.S. Department of Energy, NASA, the National Oceanic and Atmospheric Administration, and the National Science Foundation. Similar regional networks elsewhere in the world---for example, CarboEurope, AsiaFlux, OzFlux, and Fluxnet Canada---participate in synthesis activities across larger geographic areas [Baldocchi et al., 2001; Law et al., 2002].