The intrinsic behaviour of fatigue cracks is discussed by a discrete dislocation model and by a BCS-type continuum mechanics approximation. The investigation of a homogeneous material shows that the near threshold regime and the existance of an intrinsic threshold itself can be ascribed to the dislocation nature of plasticity. By incorporating microstructural features in the analysis it is then shown that the intrinsic threshold value is determined only by the mechanism for the dislocation generation and does not depend on microstructural details like the grain size. However, in the near threshold regime and in the lower Paris regime the plastic deformation and the crack growth rates are severely influenced by microstructure. Therefore, a micromechanics approach should be used for the predictions of the plastic response of materials. Only in the upper Paris regime, where the cyclic plastic zone size exceeds several times the microstructural length scale usual continuum plasticity mechanics is appropriate to describe the events at the crack tip. Macroscopic quantities like the yield stress of materials cannot be used to describe moderately stressed fatigue cracks.