Most everyday products and industrial structures are made by assembling several parts. This is the case in the aeronautics, railway, and automotive industries, where polymer composites have become increasingly popular as lightweight substitutes for metallic materials. The joining of polymer composites may be achieved using different technologies. However, one of the most significant drivers for using polymer composites, particularly thermoplastic composites, is the ability to join components via fusion bonding/welding, an attractive alternative to conventional methods, e.g., mechanical fastening and adhesive bonding. Whatever the welding technology, e.g., resistance, induction, ultrasonic, laser, or conduction welding, one of the challenges is to master the interfacial phenomena, structure, and quality in the assembly area (welds), which is rather tricky due to the presence of the reinforcement fibers. The same issues are also tackled for 3D-printed or additively manufactured parts. Additionally, there is a need for reliable predictive process simulation software, as well as in situ monitoring and control of welding process parameters. This Special Issue reprint focuses on the latest advances and development of fusion bonding/welding of polymer composites. It contains research articles addressing materials, processing, modeling, monitoring, or performance issues with either experimental or numerical approaches.