Abstract: Formation of residual stress during thermoplastic welding causes detrimental effects to the joint quality under both dynamic and static loading conditions. Residual stress can reduce the solvent resistance of polymers as well as the tensile strength and fatigue life of the joint. Therefore, it is vital to predict and measure the level of residual stresses. Here, the formation of thermal and residual stresses during implant resistance welding of polycarbonate was studied. Thermocouples and an infrared temperature sensor were used to measure the temperature history and temperature distribution in the parts during welding. Heat flow analysis during implant resistance welding was done using Finite Element Method (FEM) and Finite Difference Method (FDM) which are connected with FEM and a simplified modeling analysis so-called "multi-bar analogy" respectively for stress analysis. FEM and FDM predictions of heat flow analysis were in good agreement with experimental measurements. The formation of thermal and residual stress was predicted using 2-D finite element analysis and multi-bar analogy in conjunction with non-isothermal linear viscoelasticity for a thermorheologically simple material. The residual stresses in the parts were measured using both photoelasticity and moire interferometry. Sectioning method utilizing moire interferometry was used to measure residual stress. FEM prediction of residual stress was in good agreement with photoelasticity measurement and moire interferometry measurement. Residual stress formation in the weld was predicted by multi-bar analogy modeling analysis and multi-bar analogy prediction was in good agreement with FEM prediction. Heat treatment to reduce residual stress after welding was performed. Residual stress distribution after heat treatment was predicted using FEM. The FEM prediction was in good agreement with photoelasticity measurement and moire interferometry measurement. This methodology for prediction, measurement and reduction of residual stress can be incorporated into the design, analysis, and welding procedures for plastic and composite joints. This will result in stronger and more reliable welds.