Fracture toughness is a material’s ability to resist propagation of a preexisting crack. In most end uses where fracture,toughness can be an important performance parameter for paper, stresses are applied in the plane of the sheet. Therefore, like tensile strength and elastic modulus, the fracture toughness of paper should be measured under in ‘ -plane loading. Our current industry practice of measuring the out-of plane tearing resistance by the Elmendor for Brecht-Imset tests seems inappropriate. Several techniques of fracture mechanics have been applied in recent years to characterize the plane stress fracture toughness of paper. An important consideration is whether a material property was measured particularly for tough papers.This contribution provides a background on these techniques.