This book provides readers with an overview of recent advances in the modelling, simulation, monitoring, and optimization of complex systems for sustainable energy and industrial applications. It is organized into two core topics: renewable energy, thermal systems and sustainable water technologies, and dynamics, modeling, condition monitoring, and artificial intelligence applications in mechanical systems. Gathering peer-review contributions to the Third International Workshop on Modelling and Simulation of Complex Systems for Sustainable Energy Efficiency (MOSCOSSEE 2025), held on September 20–21, 2025, in Zarzis, Medenine, Tunisia, this book provides timely insights into emerging methods and technologies in dynamic modelling, vibration analysis, fault diagnosis, predictive maintenance and control. It serves as a valuable resource for researchers, engineers, and practitioners while fostering collaboration between academia and industry.