This Manual is the basic guideline for modelling and mapping critical levels and loads and their exceedances, and for dynamic modelling of acidification. It helps Parties to the UNECE Convention on Long-range Transboundary Air Pollution (CLRTAP) to fulfil their obligations to use harmonized methods to derive data for effects and risk assessments. This happens in the framework of the International Cooperative Programme on Modelling and Mapping Critical Loads&Levels and Air Pollution Effects, Risks and Trends (ICP M&M). This programme was established in 1988 under the leadership of Germany; it was chaired from the beginning until 2002 by Heinz Gregor who strongly influenced the development of critical levels and loads, and the role they play in European emission abatement policy.