Safe and economical design and assessment of reinforced (RC) and prestressed concrete (PC) beams requires the availability of accurate but simple formulations which adequately capture the structural response. In this paper, a mechanical model for the prediction of the shear-flexural strength of PC and RC members with rectangular, I, or T sections, with and without shear reinforcement, is presented. The model is based on the principles of concrete mechanics and on assumptions supported by the observed experimental behavior and by the results of refined numerical models. Compact, simple, and accurate expressions are derived for design and verification of the shear strength, which incorporate the most relevant shear transfer actions. Excellent agreement between the predictions of the model and the results of the recently published ACI-DAfStb databases, including more than 1,287 tests on RC and PC beams with and without stirrups, has been observed. The theory behind the model provides consistent explanations for many aspects related to the shear response that are not clearly explained by current code formulations, making it a very helpful tool for daily engineering practice. Peer Reviewed