In the design and assessment of precast concrete beams with a slab cast on top, namely concrete composite beams, engineers still face in practice unsolved shear-related issues, such as the contribution to shear strength of the slab, the concrete strength to be considered in shear formulations or the influence of the interface between concretes in the shear behaviour. In the present research, 69 shear tests were performed on monolithic and composite beams, with rectangular or T-shaped cross-section, with or without transverse reinforcement and with different concrete qualities, to experimentally analyse the issues mentioned above. The study of the shear transfer mechanisms at failure led to formulating a model for explaining the observed results. Based on this model, a shear strength predictive formulation for concrete composite beams with web reinforcement has been developed, which has been verified with the experimental results from this research and 24 additional tests from the literature. This formulation provides accurate predictions compared to the shear strength formulations of current codes EC2, MC-10 and ACI 318-19. The proposed model lays the foundations for the future development of a user-friendly formulation for calculating the shear strength of concrete composite beams.