Shear loading on structures has been recognized as one of the most relevant actions determining structural safety since the 19th century. In the case of reinforced concrete structures, despite the great efforts that have been made through experimental and theoretical research over many years, the nature of the shear failure process of a reinforced concrete beam without shear reinforcement has always, for a substantial part, remained a riddle. The present research work takes a new look at this old problem. The mechanism of flexural shear failure for a reinforced concrete beam without shear reinforcement is explained fundamentally, based on which a new failure criterion is proposed. The study leds to a shear evaluation that is in excellent agreement with test results.