The review describes monitoring of foreign compounds in forest ecosystems linked up with the international cooperative programme "ICP Forests" which is one of the programmes fulfilling the Convention on Long- Range Transboundary air Pollution ( CLRTAP). This monitoring follows the air pollutiondirectly in the forest, it measures and evaluates the deposition of selected elements and the influence of forest growth on the changes in their concentrations in soil water and in water flowing into drinking water sources. Monitoring the rainfall chemism is supplemented by analyses of selected bioindicators with a high accumulation ability, e.g. the moss Hypnum cupressiforme, organic soil horizons, leaves and needles of trees. From a long-range series of measurements the long-lasting increase in SO₂ concentrations can be seen between 1972 and 1987. Than an abrupt decrease down to the present concentrations is obvious. A similar trend is evident in the deposition of SO₄^2- by rainfalls. The nitrogen fall-out exhibited a maximum in the years 1985-1992, then it started to decrease. The accumulation of some heavy metals in forest ecosystems is still very topical.