An experimental study is reported of the behavior of reinforced concrete corbels subjected to both vertical and horizontal loads. Twenty-eight corbel specimens were tested, of which twenty-six contained horizontal stirrup reinforcement. The variables included in the study were the shear span to effective depth ratio, the ratio of the vertical load to the horizontal load, the amounts of main tensile and stirrup reinforcement, and the type of aggregate. Criteria for the deisgn of horizontal stirrup reinforcement are developed. It is shown that, subject to provision of the recommended amount of stirrup reinforcement, the useful ultimate strength of corbels can be taken to be the lesser of (a) the shear strength of the corbel-column interface, calculated using either the shear-friction provisions of Section 11.15 of ACI 318-71 or the modified shear-friction equation, and; (b) the vertical load corresponding to the development of the flexural ultimate strength of the corbel-column interface. In the next issue a follow-up paper will propose specific code provisions for designing corbels together with numerical examples.