As the U.S. moves toward registering and regulating emissions of greenhouse gases, we are hearing more about the potential for forest carbon sequestration to offset fossil fuel emissions. Whether at the national or project level, good decisions about forest offsets depend upon accurate estimates of the carbon stored in forests and the changes in those stores over time. This Brief examines four carbon measurement tools, and provides some general comparisons for broad regions. It also highlights limitations that users should keep in mind; data are particularly limited for very old forests and for carbon reserves in dead wood and underground.