The objective of this paper is to present a review of the most important scientific knowledge and uncertainties concerning forest vegetation and acidification, and to draw some conclusions for research policy. Forest vegetation and acidification are considered as parts of a global environmental system. The phenomenon of "Forest Decline" is introduced. The methods used for causal research and monitoring are presented, as well as their related difficulties. The scientific knowledge gained from epidemiological approaches (forest damage inventories), experiments and mechanistic studies are evaluated. The paper ends with some conclusions for research policy and emphasizes the need to intensify the use of integrated approaches in environmental sciences.