This book contains the abstracts presented at the 20th International Conference on Internal Friction and Mechanical Spectroscopy (ICIFMS20) held in Lausanne 7-10 July 2025. This conference is the 20th of a series that started in 1956 in Providence (USA) until the last one in Rome in 2021. It is an open window on the progress of Science in understanding the elastic, viscoelastic and viscoelastic behaviour of materials. Topics cover a wide domain of modern physics: crystal defect dynamics, magneto mechanical damping, glass transition, and very low damping materials needed for gravitational wave detection and quantum opto-mechanical devices.