The interlaminar fracture toughness in mode II and mode III of a number of advanced composites was studied using beam type test specimens and scanning electron microscopy. Special emphasis was placed on elucidating the material aspects of the fracture process and on quantifying the effect of matrix on fracture energy. The fracture energy in mode II was independent of crack extension while that for mode III exhibited a rather probablistic "resistance" behavior that was attributed to the effect of fiber bridging. The initiation fracture energy, considered here the true measure of GIII C, coincided with GII C. For either mode, the interlaminar region ahead of the crack tip exhibited considerable plastic deformations, the severity that is believed to control the laminate toughness. The interlaminar fracture energy in shear, hereby denoted as GSC (=GII C=GIII C), was accurately predicted from a straightforward adhesive joint fracture test provided the adhesive thickness coincide with the thickness of the interlaminar resin layer.