The lecture will start explaining why uncertainty assessments are important for hydrological modellers as well as for water resources managers. Then commonly used uncertainty concepts and terminologies are explained and discussed – what is uncertainty and how can it be characterized? This will be followed by an introduction to a broad range og uncertainty tools such as uncertainty matrix, sensitivity analysis, error propagation, Monte Carlo, ensemble modelling, NUSAP and expert elicitation. The tools and their application will be illustrated by examples from practical cases. At last a major case study on observational and model predictive uncertainties are presented.