This study describes experimental and numerical investigations into the inelastic behaviour of structural assemblages consisting of reinforced concrete (RC) beams connected to steel columns by means of fully embedded shear-keys. A detailed account of the experimental results and observations from a series of fourteen full-scale hybrid steel/RC specimens is presented. In order to provide further insights into the key response characteristics, particularly those related to ultimate failure conditions, a number of numerical sensitivity assessments are also carried out. The numerical studies are undertaken using nonlinear finite element procedures which are validated against previous tests on RC members as well as the experimental results from the hybrid tests presented in this paper. The ability of the numerical models to provide faithful prediction of both RC and hybrid test results, in terms of stiffness, strength and failure mode, using a consistent set of material modelling parameters, provides confidence in the reliability of the simulation techniques. Possible failure conditions for mixed RC/steel members are assessed using a suggested hybrid Mode Index (MI). Based on the experimental and numerical evaluations, simplified analytical representations of the failure surfaces, corresponding to the ultimate modes of behaviour for the hybrid configurations examined in this study, are proposed and discussed.