A national or international monitoring and assessment programme to evaluate the environmental effects of anthropogenic perturbation (e.g. acidic deposition, toxic contaminants, climate change, etc.) is best organized in an integrated hierarchical manner (represented by the left pyramid in Fig-ure 1). At the apex of the pyramid is a small number of intensively monitored process research sites. Here sufficient information is collected so that detailed dose-response relationships may be ana-lyzed, and time-dependent models developed to predict future changes in the state of the ecosys-tems. Beneath the apex regional monitoring networks are indicated which use progressively less frequent sampling at progressively more sites. The base of the monitoring pyramid is composed of 'surveys' in which sampling may occur as infrequently as once or twice per decade. Two features of the monitoring hierarchy should be noted. First, there should be some overlap be-tween the hierarchies to ensure data and model transferability among levels. Some ECE countries maintain on or more monitoring sites that contribute not only to process research but also to several ICP programmes. This is wise. Such sites are the primary source of 'ground truth' for validating and/or modifying dose-response relationships and assessment models. Second, there is an inherent assumption of the continuing existence of all levels in the hierarchy. Piecemeal, intermittent and short-term monitoring does not provide the information on temporal variations required to distin-guish natural from anthropogenically induced effects.