Principles of the statistical approach to the stability of microtortuous cracks under the remote mode I loading are presented within the framework of linear elastic fracture mechanics. The basic idea of this approach lies in the suggestion that branching occurs only at those sites along the crack front, where the characteristic structural parameter (grain size, interparticle distance) is comparable to or larger than the plastic zone size. By using procedures introduced in the paper, extrinsic contribution to the fracture toughness or to the fatigue threshold can be separated from experimentally obtained KIc or [delta]Kth values. Consequently, it enables to determine the intrinsic resistance against crack initiation and growth as a more relevant measure of materials quality. Moreover, this theory elucidates quantitatively well some interesting phenomena observed in fracture and fatigue of metallic materials.