The North American Carbon Program (NACP) plan for research on the carbon cycle is focused on measuring and understanding the sources and sinks of CO₂, CH₄, and CO in North America and adjacent ocean regions. The plan is intended as a component of the U.S. Interagency Carbon Cycle Science Program and as a contribution to the new U.S. Climate Change Research Initiative. The NACP will provide quantitative understanding of the uptake or release of these gases attributable to natural and human activity. The Program will separate the influence of combustion and biogenic sources, and determine the sensitivity of underlying biological processes to environmental conditions to biophysical and ecological factors (phenology, vegetation cover, prior land use), and to management of forests and agricultural land. The integrative framework proposed in the NACP will need to incorporate data from a wide range of sensors, locations, and processes and to connect measurements obtained at multiple scales with regional and continental scale data. AmeriFlux is a coordinated research network of long-term flux sites in the Americas for quantifying and understanding the role of the terrestrial biosphere in global climate change. The AmeriFlux Program uses the eddy covariance method to estimate net ecosystem exchange (NEE) of CO₂, water vapor and energy between vegetative and the atmosphere at scales from several hectares to kilometers. There are about 60 AmeriFlux sites, and roughly half of them have been operational since 1996. Because disturbance results in significant changes in carbon storage and fluxes, efforts are moving towards quantifying storage and fluxes in different age classes of forests and landscapes with various disturbance histories. Based on a number of years of measurement, results from many sites show terrestrial ecosystems are gaining carbon at an average rate of several tones per hectare per year. AmeriFlux data are used in carbon models to evaluate and improve estimates of ecosystem productivity, net carbon uptake and carbon storage across broad geographic areas of N. America. AmeriFlux and the NACP will provide the scientific foundation for predicting large-scale, long-term carbon responses to changing environmental conditions.