Multi-national statistics are frequently based on data, which originate from national surveys. The systems of nomenclature applied for key attributes often show national differences. Different error sources which are incorporated in multi-national statistics are discussed. The paper presents approaches for harmonisation and standardisation of multi-national environmental statistics and gives examples from the forestry sector. The effect of differences of national forest area estimates on multi-national figures is quantified. An example from forest health surveys is presented that shows the impact of different interpretation and application of the attribute "crown transparency" that is already harmonised on the European level.