Reduced models are important for testing and optimizing the designs of permanent and temporary hydraulic structures. Until the end of the nineteenth century, hydraulics was basically empirical, to the extent that it was defined as the “science of coefficients.” At the end of the eighteenth century and during the nineteenth century there was remarkable experimental work on flow in hydraulic conduits and in singularities. Parallel to the evolutive aspects, another notable occurrence was recorded, namely the use of two mathematical tools of work, the theories of dimensional analysis and mechanical similarity, discovered by Newton, which resurfaced as powerful levers and boosted the experimental technique in the field of hydraulics. In the design of hydraulic structures models, the following considerations need to be made: the desired results; the available space for model construction; the supply of water; and the cost. The design of river channel models presents more difficulties than models governed only by Froude’s law.