Fish passage structures are built to restore the connectivity of rivers and allow the migration of aquatic fauna. In order to assess the functioning of a pool-type fishway, it is necessary, inter alia, to possess detailed knowledge of its flow structure, since observations of fishways, and in particular of the visible water surface, can only provide a rough idea of the actual conditions inside the pools. Numerical simulation has been used for many years to support engineering sciences. Especially, the modeling of flow processes in hydraulic machines can, on the one hand, help avoid major problems during the design stage of fish passage structures and, on the other, improve the structure's hydraulic performance. To this end, two diploma theses within the framework of a research project of the local energy supplier Energie Baden-Württemberg AG (EnBW) employed modeling tools for 3D flow simulation, primarily for pool-slot fishways (PSF), and for traditional vertical slot fishways (VSF).