Reinforced concrete (RC) grid slab systems are commonly used to cover large column free spaces. This system of slabs and grid beams is predominantly subjected to flexure, shear and torsion under the action of uniformly distributed gravity loads. Conventionally, the design of the slab portion is done using the simplified moment coefficients prescribed in various codes, assuming the supports to be non-deflecting (rigid supports), considering appropriate boundary conditions at the four edges of each slab panel (continuous or discontinuous). The supporting beams are analysed, assuming load transfer from the slab panels using tributary area concepts. The failure pattern typically assumed for the slab corresponds to a ‘slab-alone failure’ mechanism, for which the collapse load can be estimated using yield line theory; enhancement in the actual collapse load observed in experiments is attributed mainly to tensile membrane action. When the supporting beams are flexible (as in a waffle slab), the system is usually analysed (under factored loads) using linear elastic theory (as in standard finite element softwares). This paper shows how the yield line theory can be applied to all beam-slab systems, accounting for the relative flexibilities and flexural strengths of slab and beam components. The failure mechanism can occur either by ‘slab-alone failure’ or by ‘combined beam-slab failure’. The latter involves yielding of the longitudinal tensile reinforcement in the beams which intercept the yield lines. The focus of the paper is on developing a formulation to predict the collapse load of simple rectangular beam-supported slab systems; this can be extended to larger rectangular grid systems. The results have been validated with experiments reported in the literature on square beamsupported slabs. © 2016, A.A. Balkema Publishers. All rights reserved.