This book offers a collection of original peer-reviewed contributions presented at the 9^th International Congress on Design and Modeling of Mechanical Systems (CMSM’2021), held on December 20-22, 2021, in Hammamet, Tunisia. It reports on research findings, advanced methods and industrial applications relating to mechanical systems, materials and structures, and machining. It covers vibration analysis, CFD modeling and simulation, intelligent monitoring and control, including applications related to industry 4.0 and additive manufacturing. Continuing on the tradition of the previous editions, and with a good balance of theory and practice, the book offers a timely  snapshot, and a useful resource for both researchers and professionals in the field of design and modeling of mechanical systems.