Abstract <p>The paper analyzes the stages and kinetic features of fatigue crack growth. Particular attention is paid to fatigue stage II consisting of two substages, namely, II_a and II_b. The stress intensity factor <i>K</i>_S is proposed to determine the boundary between them (corresponds to the stable crack length <i>a</i>_S under plane-strain conditions) and to characterize the cyclic fracture toughness. It is assumed that <i>K</i>_S corresponds to the stress intensity factor <i>K</i>_GY estimated by the cyclic yield stress and the length of a focal fatigue crack. Enlargement of the plastic zone at the crack tip and a transition to the plane-stress state at <i>K</i> ≥ <i>K</i>_S change the fatigue fracture pattern, which manifests itself as knee points in the <i>K</i>_max dependences of acoustic emission parameters, phase transformation rate in metastable steel, and fatigue striation spacing: after reaching <i>K</i>_S, the fatigue crack grows by the striation-per-cycle pattern. In addition, it is shown that the value of <i>K</i> = <i>K</i>_S corresponds to the pivot point of the crack growth curve plotted for the steel tested in mixed loading modes.</p>