Forests play an important role in the U.S. and global carbon cycle. Carbon sequestration by U.S. forest growth and harvested wood products currently offsets 12-19 percent of U.S. fossil fuel emission. The cycle of forest growth, death and regeneration and the use of wood removed from the forest complicate efforts to understand and measure forest carbon pools and flows. This report explains these processes and examines the science behind mechanisms proposed for increasing the amount of carbon stored in forests and using wood to offset fossil fuel use. We also examine the tradeoffs, costs and benefits associated with each mechanism and explain how forest carbon is measured.