The most important factors to consider for a complete determination of the toxicity to vegetation of acidic precipitation are: settling velocity, mass transfer rates, capacity to wet surfaces; number, frequency, and duration of events; and chemical composition and concentration. Meteorological factors, such as wind velocity and humidity, also must be considered because they affect the physical, temporal, and chemical properties of precipitation. Climate is important because it influences the capacity of plants to adjust, recover, or compensate for exposure to acidic precipitation. Genotype affects the efficiency of capture and capacity to tolerate acidic conditions. Aerosol and fog droplets generally are more acidic than mist and rain but they may be less phytotoxic because smaller droplets are deposited and captured less readily by leaf surfaces except when wind velocity is high. Concentrations of acids, mainly sulphuric and nitric acids, in aerosols, fog, mist, and rain in polluted regions, appear to be insufficient to produce acute injury on vegetation except perhaps in the immediate vicinity of intense sources of emissions. Chronic effects of repeated exposure to acidic precipitation, such as the impact on plant nutrition or on processes occurring at the leaf surface-atmosphere interface, and interactions with gaseous pollutants such as ozone, cannot be evaluated at present owing to the lack of information.