Ozone is a secondary air pollutant formed under conditions of bright sunlight and warm temperatures as a result of photochemical reactions involving nitrogen oxides and hydrocarbons as primary precursor pollutants. Tropospheric ozone is of regional-scale importance due to long-range transport of polluted air masses from urban and industrial areas of the Midwest and Northeast into agricultural and forested regions situated downwind of these expansive source regions. Ozone is perhaps the most important air pollutant of concern to the long-term productivity and health of trees and other native plants which comprise our natural and forested areas of the Northeast. On broadleaf species, foliar injury is exhibited as a stipple and/or more general pigmentation of the upper leaf surface late in the growing season. These symptoms are considered to be the best response parameter to observe and relate to ambient ozone exposures; ozone-induced injuries on conifers are more difficult to diagnose. Several plant species are known as bioindicators of ozone exposures; symptom observations from a recent open-top chamber investigation in central Pennsylvania confirm black cherry, yellow poplar, white ash, common milkweed, spreading dogbane, and blackberry to be sensitive to ambient ozone exposures. Concerns for lack of ozone air pollution monitoring in remote forested areas and more complete understanding of the effects to tree species and other native plants are discussed.