Most everyday products and industrial structures are made by the assembly of several parts and therefore contain joints. This is the case for example in the aeronautics, railway, or automotive industries where polymer composites become more and more popular as lightweight substitutes of metallic materials. Joining of polymer composites may be achieved by different technologies, as in the case of metals. However, one of the greatest drivers for thermoplastic composites use is the ability to join components via fusion bonding/welding, which is an attractive alternative to conventional methods—mechanical fastening and adhesive bonding—used to join thermoset composite parts. Although some methods such as resistance welding or induction welding are quite well established, other technologies, including ultrasonic welding, laser welding, infrared welding or conduction welding, are still at a more or less advanced development stage. 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 to tackle for 3D-printed or overmolded parts. Besides, there is a need for reliable predictive process simulation softwares, and also for increased inline monitoring and control of welding process parameters. This Special Issue gathers papers on the latest advances and development of fusion bonding/welding of thermoplastic composites (i.e., fiber reinforced thermoplastics) addressing materials, processing, modeling/simulation, monitoring/control, performance or application issues, with either experimental or numerical approches.