This chapter delves into the science of forests and streamflow. The amount of water that runs off the land into a stream is known as water yield. It is the water available for human uses. Evapotranspiration is a loss of water that reduces streamflow. Forests increase annual evapotranspiration and reduce annual streamflow compared with grasslands and other types of vegetation. The science, however, is not precise and our understanding is more qualitative than quantitative. Water yield and the climate services of forests represent conflicting demands for water. Forests cool the surface climate through evapotranspiration and remove carbon dioxide from the atmosphere during growth, but in doing so they consume water and reduce water yield for human usage. Consideration of spatial scale further muddies the policy implications because any precipitation benefits of forests occur at large spatial scales covering vast regions of land or entire continents, while the water cost of forests is felt at the scale of the watersheds that supply towns and cities.