Models are used to both make predictions about the future of natural systems and to gain a better understanding of current conditions. Their use in the field of water resources is widespread, for simulating both the quantity and quality of freshwater systems. Models will undoubtedly continue to serve a vital role in tackling our water challenges in the future. That said, models must be used smartly and appropriately in all applications. Good modellers understand the limitations of their tools. They also understand the value added, and lack thereof, by model complexity and work to ensure ‘fit for purpose’ in their developed models. In this paper Dr Tim Cox discusses the subjects of model uncertainty, transparency, simplicity and usability in this discussion paper and provides some ‘food for thought’.