As all hydrological models are intrinsically limited hypotheses on the behaviour of catchments, models – which attempt to represent real-world behaviour – will always remain imperfect. To make progress on the long road towards improved models, we need demanding tests, i.e. true <i>crash tests</i>. Efficient testing requires large and varied data sets to develop and assess hydrological models, to ensure their generality, to diagnose their failures, and ultimately, help improving them.