The assessment and evaluation of tree condition and vitality is an essential part of the ICP Forests monitoring programme. Due to the complex structure of forest ecosystems, a number of different indicators of tree condition are adopted. In this chapter, they are described also in terms of their ecological relevance. Special emphasis is laid on biotic agents affecting forest ecosystems and explaining tree responses to stressors. For operational reasons, definitions and recommendations for the use of the indicators are given. Tree condition assessment in forest monitoring is closely linked to field measures of Quality Assurance and Control. Examples of results show that the methods currently used in tree condition monitoring are sensitive to detect the effects of climate change on forests and identifying main spatial and temporal patterns and damaging agents.