This book gathers selected, peer-reviewed contributions to the international conference on Water Jet 2023 (WJ2023), organized by the Institute of Geonics of the Czech Academy of Sciences. It covers research and developments of water jets, and their applications in cutting, machining, cleaning, surface treatment, and milling. Further topics include the use of water jets in the removal of coatings and layers, jet grouting, jet observation and parameters measurement. Moreover, erosion wear, environmental aspects of water jets and their improvement with particular emphasis on sustainable practices, are also discussed. Overall, this book provides academics and practitioners with extensive information on state-of-the-art theories and methods that can be used to understand, analyse and improve the rapidly growing field high-velocity water jet technology.