Declines in oak-forest ecosystems have been reported frequently throughout the world since the beginning of this century. They have been associated 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 conditions in selected countries. Oak decline is regarded as a result of the synergistic action between damaging agents, both biotic and abiotic. 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 influence on the vigour of oaks. An increase in the frequency of dry years (especially when drought occurs at the beginning of the growing season, e.g. in May-June) has contributed to the complex phenomenon of oak decline.