Hydrological processes can be well studied in small research basins, where high-quality data are available. They serve as interdisciplinary observatories or outdoor laboratories covering the complexity of combined physical, chemical, and biological processes. Furthermore, long-term measurements in small research basins help to analyse environmental changes. New measurement and sensing techniques support further hydrological understanding. For the depiction and understanding of hydrological processes, numerical modelling approaches can additionally be used. On the one hand, hydrological models need sufficient data with high spatial and temporal resolution for model set-up, calibration, and validation. On the other hand, they offer the possibility of predicting processes that are not easily measurable, and allow for the calculation of the impact of supposed scenarios that will possibly affect the water cycle and mass transport in the future. Small research basins provide the high-quality data required for model development and testing, also for new and innovative model approaches. Knowledge gained from monitoring and modelling studies in small hydrological basins can be used for decision-making with respect to managing water resource systems.