The use of punching shear reinforcement is currently considered as one of the most convenient practices to enhance the punching shear strength and deformation capacity of slab-column connections in flat slabs. Intensive research has been performed in the last decades on this topic, evidencing the complexity of the phenomenon, which is dependent on the anchorage conditions and detailing rules of the punching shear reinforcement, as well as on its capability to control cracking in the shear-critical region. Despite these efforts, the design of punching shear reinforcement, and particularly the verification of the maximum punching resistance, relies still on several empirical coefficients which enhance the calculated capacity of slabs without shear reinforcement. These methodologies require experimental validation, and no predictions or optimization can be performed on a scientific basis. In an attempt to advance on a rational approach to the design of punching shear reinforcement, this paper introduces a novel methodology based on the fundamentals of the Critical Shear Crack Theory. By investigation of potential failure surfaces, the governing shape of the critical shear crack is determined and the contributions to the total resistance of concrete, flexural reinforcement and punching shear reinforcement are calculated accordingly (based on the slab rotations and the column penetration). This method is shown to be comprehensive and to consistently explain experimental evidence when compared to selected tests as well as general databases. The model is eventually used to show the role of a number of variables in the maximum punching strength behaviour of slab-column connections in flat slabs, including column size, detailing rules of punching reinforcement (spacing and anchorage performance), size effect and other parameters related to the level of strains in the slab (yield strength, amount of flexural reinforcement and slab slenderness).