The hydraulic model is playing an increasing part in the process of hydraulic designing and it seems even to have taken over the dominant position that experience and tradition had held for such a long time. But, however important the model tests have become, they are not the only means available to the designer, nor does he have to rely solely upon them. Experience still holds a prominent place, while investigations in situ procure for the designer basic information he seldom can dispense with. A close interaction between investigation in model and prototype is a condition for obtaining reliable results. Beside these relations in the hydraulic field, a design is influenced by other considerations, such as in the fields of technical construction and economy. The model (hydraulic, mathematical or other) has its own place between several other components of the design. As already stated, the influence of the hydraulic model in this interaction is increasing and is now tending to become the central point in the process of designing. This is to a large extent due to progress in model techniques: simulation, even of very intricate hydraulic phenomena, has been greatly improved by experience as well as through basic research, while also the instrumentation of the laboratoryis steadily improving.