This book contains the results and conclusions of extensive research on the causes and effects of acidification of forests/forest soils and heathland in the Netherlands. In 1985 the Dutch Priority Programme on Acidification was started in order to give a more concrete form to the increasing interest of policy-makers in the effects of air pollution on ecosystems in particular. In the last three years, the research has focused on obtaining a more accurate estimate of the emission of ammonia on the deposition of SOx, NOy and NHx, and also on quantifying effects on forest and heathland ecosystems. This quantification of effects included experimental work model analyses, and derivation of critical loads and levels for forest and heathland ecosystems. Furthermore, scenario analyses were made with the Dutch Acidification Systems Model (DAS) in order to evaluate the effectiveness of policy measures. The research itself, which formed the basis for this book, has been described in the reports on individual projects. A summary of the scientific results and conclusions is given in thematic reports (added as annex). An independent Review Team has provided a critical assessment of the research carried out since 1988. The Review Report has been incorporated as annex.