The fracture toughness perpendicular to the reinforcement of a composite is much greater than the volume fraction average of the fracture toughnesses of its components. Several years ago Outwater and Mandell independently proposed that the high toughness of glass reinforced plastics is due to the tensile energy stored in the debonded region for the fibers in the vicinity of the fracture surface. Using appropriate modifications of Irwin's expression for the strain energy release rate, this concept is shown to be valid in other composite systems as well. Analogies also extend its use to composites with ductile fibers and elastoplastic matrices. The data imply that the toughness contribution of the fiber to the overall composite toughness is proportional to the irrecoverable energy in the fiber, that is, the elastic and plastic energy within a debonded region of the fiber or within a portion of the fiber encased by matrix material that has yielded. The results obtained here, in conjunction with the Outwater-Mandell toughening mechanism, lead to a unified concept of toughening in fiber reinforced materials.