For about a decade, model-based reasoning has been propounded by a number of researchers. Maybe one of most convincing arguments in favor of this kind of reasoning has been given by Davis in his paper on diagnosis from first principles (Davis, 1984). Following his guidelines we have developed a system to verify the behavior of a satellite-based instrument GOME (which started measuring ozone concentrations in 1995). We start by giving a description of model-based monitoring. Besides the ability to recognize that something is wrong, we also desire the ability to automatically identify the cause of the misbehavior. Therefore, we show how the monitoring technique can be extended to model-based diagnosis.