The fracture toughness of poly(ethylene terephthalate) (PET) is correlated with various morphologies produced upon annealing. Annealing PET at a Hoffman regime III crystallization temperature (120-degrees-C) or inducing solid-state polymerization result in materials of high fracture toughness (K(c) = 8.7 MPa m1/2 and 9.5 MPa m1/2, respectively); these high toughness values are related to multiple-crazing mechanisms produced by high tie-molecule densities. Annealing fully dried PET at 200-degrees-C transforms the material to a Hoffman regime I/II structure, and the fracture toughness decreases to 6.5 MPa m1/2. Hydrolysis also reduces the fracture toughness (K(c) < 3 MPa m1/2). In these low toughness materials, multiple crazing is not observed. In addition, the fatigue crack propagation response of PET is found to be superior in samples annealed to produce high fracture toughness as compared with samples annealed to produce low fracture toughness values.