Tropospheric ozone (O(3)) was first determined to be phytotoxic to grapes in southern California in the 1950s. Investigations followed that showed O(3) to be the cause of foliar symptoms on tobacco and eastern white pine. In the 1960s, "X" disease of ponderosa pines within the San Bernardino Mountains was likewise determined to be due to O(3). Nearly 50 years of research have followed. Foliar O(3) symptoms have been verified under controlled chamber conditions. Studies have demonstrated negative growth effects on forest tree seedlings due to season-long O(3) exposures, but due to complex interactions within forest stands, evidence of similar losses within mature tree canopies remains elusive. Investigations on tree growth, O(3) flux, and stand productivity are being conducted along natural O(3) gradients and in open-air exposure systems to better understand O(3) effects on forest ecosystems. Given projected trends in demographics, economic output and climate, O(3) impacts on US forests will continue and are likely to increase.