Fracture toughness of aluminum-lithium 8090-T8 alloy has been investigated. J-R and K-R curves tests have been performed on compact tension (CT) samples machined from a 12-mm-thick plate, both in longitudinal transverse (LT) and transverse longitudinal (TL) directions. Sample thickness and width varied in order to assess the dependence of constraint on the geometry. It was found that, owing to delaminations occurring perpendicular to short transverse direction and local fracture path deviation from Mode I, a relaxation of the degree of thickness constraint inside the material takes place. Large pop-in phenomena ensue, thus hampering the J-R curves interpretation. Metallographic structure and fracture surfaces were investigated to clarify the micromechanisms of fracture and to ascertain the possibility to single out unequivocally the critical event for JIc significant determinations. Also, K-R curves have been explained on the basis of the results of fracture mechanisms and microstructure. The extent to which JIc and K-R curves are representative and size independent, as well as the inapplicability of ASTM Test Method for Plane-Strain Fracture Toughness of Metallic Materials (E 399-83), have been discussed.