Residual stresses in molded thermoplastics are caused by heterogeneous cooling and by different pressures frozen in at different time of the cycle. This usually leads to a core in tension, surface regions in compression, and sometime thin extreme surface layers in tension again. In a first part, the mechanisms of residual stresses formation are presented. In a second part, a technique to measure residual stresses, based on the measurement of the bending moments necessary to lay-flat a specimen after removal of a thin layer, is applied to unfilled and fiber reinforced materials with success. The technique, unlike the curvature measurement based method, eliminates the necessity of the assumption of constant material properties. Finally we show the dependence of the solid polymer viscoelastic data on the thermomechanical history of the test specimen, and the consequences on the computed residual stresses, which are compared to the measurements.