In this project the basic mechanism of resistance of reinforced concrete beams subjected to flexure and shear has been investigated. From an examination of the equilibrium conditions for the shear span of a beam it was shown that shear can be resisted in two different ways, namely by beam action and by arch action. With beam action bond forces are induced in the shear span and these act on the concrete blocks lying between the flexural cracks so that bending moments are induced in the blocks. There are three possible ways in which such moments may be resisted, namely, by the flexural resistance of the concrete between the cracks, by shear transfer across the cracks due to aggregate interlock and dowel actions, and by truss action which depends on the inclusion of shear reinforcement in the beam. To investigate the different actions eight rectangular beams were tested, and tests were made on numerous small concrete specimens. In the small scale tests fundamental aspects of aggregate interlock and dowel action were examined , and in several of the beam tests the proportions of the shear resisted by individual actions were determined. The other beams were tested to observe the way in which deformations developed in the shear span, and the way the cracks opened. It was found that prior to diagonal cracking of the shear span beam action predominates, and only a very small component of the shear may be resisted by arch action. In beams with small a/d ratios arch action may develop after diagonal cracking of the span, but in beams with higher a/d ratios the occurrence of this action may lead to immediate failure of the beam. In typical rectangular beams without web reinforcement it was found that the bond force moment acting on the concrete blocks between the flexural cracks could be resisted in approximately the following proportions; (i) 20 per cent by the flexural resistance of the concrete, (ii) 60 per cent or more by aggregate interlock action, and (iii) 20 per cent or less by dowel action of the reinforcement.