In this lecture on Solid-State Joining: Mechanisms and Processes, we cover bonding mechanisms like diffusion, recrystallization, mechanical interlocking, and controlled localized melting. Processes such as friction stir welding, friction welding, ultrasonic welding, diffusion bonding, and roll bonding are discussed, along with principles, key parameters, and joint microstructures. The role of tool design, heat generation, deformation, and process control in achieving strong, defect-free joints is highlighted.