Declines in oak-forest ecosystems have been reported frequently throughout the world since the beginning of this century. They have been assoeiated with the death of individual trees, of small groups of trees or of entire stands. In this paper, the phenomenon of oak decline is presented in the context of the local site eonditions in seleeted eountries. Oak deeline is regarded as a result of the synergistic aetion between damaging agents, both biotie and abiotie. Periodic changes of the climate in selected experimental plots of Quercus robur stands in Poland were analysed by comparing Walter's climatic diagrams and annual growth rings. Climatic conditions have a major intluenee on the vigour of oaks. An increase in the frequency ot dry years (espeeially when drought occurs at the beginning of the glowing season, e.g. in May-June) has eontributed to the eomplex phenomenon of oak deeline.