Upcoming decades will experience increasing atmospheric CO2 and likely enhanced O-3 exposure which represents a risk for the carbon sink strength of forests, so that the need for cause-effect related O-3 risk assessment increases. Although assessment will gain in reliability on an O-3 uptake basis, risk is co-determined by the effective dose, i.e. the plant's sensitivity per O-3 uptake. Recent progress in research on the molecular and metabolic control of the effective O-3 dose is reported along with advances in empirically assessing O-3 uptake at the whole-tree and stand level. Knowledge on both O-3 uptake and effective dose (measures of stress avoidance and tolerance, respectively) needs to be understood mechanistically and linked as a pre-requisite before practical use of process-based O-3 risk assessment can be implemented. To this end, perspectives are derived for validating and promoting new O-3 flux-based modelling tools.