Fiber attrition during processing of long glass fiber-reinforced polypropylene at industry relevant fiber concentrations was studied experimentally to obtain a fundamental understanding of the physics in fiber breakage. The developed experimental setup of a Couette rheometer allows to study fiber breakage under highly controlled conditions in a simple shear flow. In combination with a novel fiber length measurement technique, the outcome of this work provides repeatable, accurate and robust measurement data quantifying the fiber length reduction at various process conditions. The experimental study aims to establish a theoretical link between processing conditions and fiber breakage of long glass fiber-reinforced thermoplastics. The impact of fiber concentration, initial fiber length, residence time, melt temperature and processing speed was quantified and studied. Even for the gentlest processing conditions, the results indicate that the residual fiber length after processing is less than 50% of the initial fiber length. For the most severe processing conditions, the results suggest a reduction to less than 10% of the initial fiber length, which highlights the challenges that fiber breakage poses for processing of long glass fiber-reinforced thermoplastics. The design of experiments aimed to isolate the effect of process variables on fiber breakage and their relative significance was determined. Lastly, the obtained data were used to test the Phelps-Tucker fiber breakage model, which assumes a buckling type failure of the fibers during processing. The continuum model was applied for all Couette rheometer experiments and the three empirical model parameters were fitted to the measurements. While it was possible to match simulation and measurements for all runs by adjusting the model parameter, the model parameters failed to show a material dependency of the parameters and it was not possible to compute them a priori.