Modeling in fluid mechanics, hydraulics, and hydrology, whether using digital tools or using scale models, has reached sufficient maturity to be in daily use by engineers for analysis and for communication. Increasingly complex cases can be handled thanks to ever more sophisticated tools and increasingly abundant computing power. However, quantifying uncertainty in the data and in the models for integration into decision-making processes remains a legitimate concern. With respect to these issues, a number of questions remain open: the calibration and accuracy of complex models, their relevance to case studies, and their prediction capability.