Within the framework of updating the release and storage capacities of existing reservoirs, the use of piano key weirs (PKW) can be an interesting alternative either to increase the discharge of a free weir without changing the weir width on the dam crest nor the reservoir level, or to increase the threshold reservoir level before releasing water while keeping the weir width and the level of safety. Results of first experiments on scale models have shown the high efficiency of PKW in terms of discharge capacity. But, even if PKW projects start to be developed on real structures, lacks persist in the understanding of the flow behavior on this new type of weir, in the characterization of their hydraulic potential and in the availability of design guidelines. This paper presents the results of several research projects, based on both numerical and physical modeling, carried out on this topic at the Laboratory of Engineering Hydraulics of the University of Liege. The researches rely on physical experiments on scale models of idealized PKW elements or real structures, as well as on numerical modeling with specifically developed solvers. The paper presents an overview of these projects, emphasizing the most important results regarding PKW design and performance.