This article describes the conceptual bases for a sound monitoring of pesticides in surface waters. The concept helps to decide where and when which suite of pesticides should be measured. This suite depends on the pesticides that are used in any given catchment. Therefore, the paper presents GIS-based estimates of where which type of contamination is tobe expected. Based on a statistical farmer survey, a list of priority substances is presented. Apart from use data it accounts for substance properties. lt is also essential to measure stream discharge. A simple mathematical approach is described that allows deriving the concentration dynamics from discharge.