Currently, there is no generally-accepted theory giving a physical explanation of the shear strength in one- and two-way slabs. Furthermore, for members without transverse reinforcement, shear strength is estimated in most codes of practice following empirical or semi-empirical approaches. In this paper, the fundamentals of the Critical Shear Crack Theory (CSCT) are introduced. This theory, based on a mechanical model, is shown to provide a unified approach for one- and two-way shear in slabs, leading to simple design expressions for estimating the strength and deformation capacity of such members. The paper also details a code-like formulation based on this theory and developed for the Swiss code for structural concrete. Comparisons of the theory to a wide range of test data are finally presented.