Ozone, one of the major atmospheric pollutants, alters tree growth, mainly by decreasing carbon assimilation and allocation to stems and roots. To date, the mechanisms of O-3 impact at the cellular level have been investigated mainly on young trees grown in controlled or semi-controlled conditions. In the context of climate change, it is necessary to introduce a valuable defence parameter in the models that currently predict O-3 impact on mature trees and the carbon sequestration capacity of forest ecosystems. Air pollution is an important factor that affects negatively forest ecosystems. Among oxidative air pollutants, ozone is considered as the most toxic in terms of impact on vegetation. This paper focuses on the negative impacts of ozone on trees in controlled conditions or in their natural environment. The current knowledge of the responses at cell level is presented and ways to improve their use for ozone risk assessment of forest stands are discussed. Information was collected from original papers or reviews, providing an overview of the research conducted over the last 60 years. The negative effects of ozone on carbon assimilation and tree biomass production were reviewed and discussed, with a focus on effects on cell processes implied in cell defence, including stomatal regulation, detoxification, signalling, and biosynthesis of wood compound. In the context of increasing significance of O-3 flux approach, this review intends to shed light into the black box of defence processes, which are playing a crucial part within the effective O-3 dose modelling. Today, it is recognized that tropospheric ozone inhibits tree growth and its role on the future carbon sink of the forest ecosystem is discussed along with the combination of other environmental factors like elevated temperature, water, and nitrogen supply, likely to be modified in the context of climate change.