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Abstract: Early studies of plant response to ozone (O3) utilized concentration-based metrics, primarily by summarizing the commonly monitored hourly average data sets. Research with the O3 concentration parameter led to the recognition that both peak concentrations and cumulative effects are important when relating plant response to O3. The US and Canada currently use O3 concentration-based (exposure-based) parameters for ambient air quality standards for protecting vegetation; the European countries use exposure-based critical levels to relate O3 to vegetation response. Because plant response is thought to be more closely related to O3 absorbed into leaf tissue, recent research has been focused on flux-based O3 parameters. Even though flux-based indices may appear to be more biologically relevant than concentration-based indices, there are limitations associated with their use. The current set of flux-based indices assumes that the plant has no defense mechanism to detoxify O3. This is a serious limitation. In this paper, we review the literature on exposure- and flux-based indices for predicting plant response. Both exposure- and flux-based metrics may overestimate plant response. At this time, flux-based models that take into consideration detoxification mechanisms (referred to as effective flux) provide the best approach to relate O3 to plant response. However, because there is considerable uncertainty in quantifying the various defense mechanisms, effective flux at this time is difficult to quantify. Without adequate effective-flux based models, exposure-based O3 metrics appear to be the only practical measure for use in relating ambient air quality standards to vegetation response. [Copyright &y& Elsevier]