This lecture covers the fundamentals of polymer welding, including techniques such as glueing, mechanical fastening, and welding of plastics. It explains the characteristics of thermoplastics, the process of chain diffusion, and the differences between amorphous and semicrystalline polymers. The lecture also delves into the challenges of welding incompatible polymers and the use of block copolymers as compatibilizers. Practical aspects of welding, such as the development of strength at the interface and reaction bonding, are discussed.