Part I of the present thesis deals with crack formation in reinforced concrete and the phenomenon of tension stiffening in concrete tension rods reinforced with deformed bars.Two physical models are presented for uniaxial tension, and they are modified for application on beams subjected to pure flexure.In the first model, the yield zone model, it is assumed that the mean crack distance is a descending function of the reinforcement stress in a crack. Furthermore it is assumed that in certain zones between the cracks the concrete is carrying its full effective tensile strength, i.e. the concrete is yielding.In the second model, the concrete crack stress model, the mean crack distance is assumed to be constant from the very initiation of cracking. In this model it is assumed that there is a certain transmission of concrete tensile stresses in the cracks.The stress-strain diagrams and mean crack widths predicted by the models are compared with experimental data from tests on tension rods as well as flexural beams.In the light of the simple assumptions made and the random nature of cracking, the accordance between the models and the test data is quite good.Part II of the present thesis deals with deformations in reinforced concrete disks subjected to pure shear.A physical model for the shear stress-shear strain behaviour of disks, including tension stiffening, is proposed.In the disk model it is assumed that the tensile principal stress in the concrete decreases linearly from the initiation of cracking until a certain load level. At any load level the model can predict the shear strains of the disk and the inclination of the crack system. When regarding tension stiffening this latter parameter will be a function of the load level.The model is compared with experimental data, and in the light of the simple assumptions, quite good accordance is found.Part III of the thesis deals with the deformations of a beam symmetrically loaded by two concentrated forces. In the shear-flexure beam model it is assumed that the load is carried by means of a stringer system and a diagonal stress field in the shear spans. In the shear spans the principal tensile stress of the concrete is assumed to decrease linearly from the initiation of cracking until a certain load level. The deflections of the points of application of the loads is determined, as is the inclination of the cracks. As was the case in the disk model, this latter parameter will be a function of the applied load.The model is compared with experimental data, and surprisingly good accordance is obtained.