As American cities expanded during the Gilded Age, city planners pushed for tree-lined streets and leafy parks as public health measures. In overcrowded tenement districts where heat could be deadly, trees cooled the air. Today, researchers are discovering that urban dwellers who live near and among trees have healthier babies, display better cognitive functioning, and report less stress—no small matter when 80 percent of Americans live in and around cities. City trees and woods—which clean the air, cool urban heat islands, and capture stormwater runoff—can and should play a more important role.