In the first part of this chapter the size effect in concrete is studied by means of finite element analysis. The rheological material model developed at TH Darmstadt was used for a parameter study. The strength of the concrete and the shape of the stress-crack opening relation was varied. The increasing flexural strength of concrete with decreasing depth of the beams is described. It is caused by the strain-softening behaviour of concrete. The effect of a normal force acting simultaneously with a bending moment is also taken into account. The outcome is that size effects also depend on eccentricity and material properties. In the second part the rheological material model was used to analyse three-point bending specimens of four different sizes. The measured crack-mouth-opening curve was used to recalculate the stress-crack opening relation (σ–w relation). It is shown that the same σ-w relation is able to describe the specimen behaviour independent of the size. With the unique σ–w relation found for the concrete used, the peak loads of the whole range between very small and very large specimens were calculated and compared with a yield criterion and with linear elastic fracture mechanics.