The article states the negative influence of immissions on the state of health of forests. The extent of forest damage in Europe determined within the frame of International cooperational programme of evaluation and monitoring of the effects of air pollution on forests is mentioned. According to it in Slovakia has been damaged by different intensity 85% of forests. The aim, resp. the content of international monitoring and also the classification of damage of forest by immissions according to defoliation and decoloration of assimilative organs is characterized in more detail. In Slovakia the Research Institute of Forest Management in Zvolen deals with the monitoring of the state of health of forests – it has developed the network of 111 permanent monitoring areas.