Practitioners of hydraulic structures engineering find nonlinear weirs attractive because they can provide a hydro-economical solution for rehabilitation projects. Their complex flow patterns, wide range of geometries and applications have also contributed to their appeal in academia. In a world increasingly defined by ageing infrastructure, diverse project goals and hydrological extremes, nonlinear weirs, in particular labyrinth and piano key weirs, have emerged as a desirable option for construction.This proceedings compilation presents the latest research results and innovations in the field on labyrinth and piano key weirs. Each contribution has been reviewed and selected by the International Scientific Committee of the 4thInternational Workshop on Labyrinth and Piano Key Weirs (7-9 October 2025, Atlanta, Georgia, USA). References to projects at the design and under-construction stage have also been included.This publication is a useful guide for students, researchers and professionals interested in dams engineering, civil engineering and hydraulic structures engineering.