Static and cyclic end-notched flexure (ENF) tests were conducted on three materials to determine their interlaminar shear fracture toughness and fatigue thresholds for delamination in terms of limiting values of the mode II strain energy release rate, GII, for delamination growth. Data were generated for three different materials: a T300/BP907 graphite/epoxy, an S2/SP250 glass/epoxy, and an AS4/PEEK (polyetheretherketone) graphite/thermoplastic. The influence of precracking and data reduction schemes on the mode II toughness and fatigue behavior is discussed. Finite-element analysis indicated that the beam theory calculation for GII with the transverse shear contribution included was reasonably accurate over the entire range of crack lengths. However, compliance measurements for the three materials tested and the variation in compliance with crack length differed from the beam theory predictions. For materials that exhibited linear load-deflection behavior, GIIc values determined from compliance calibration measurements provided the most conservative and accurate estimate of the interlaminar shear fracture toughness. Cyclic loading significantly reduced the critical GII for delamination. A threshold value of the maximum cyclic GII below which no delamination occurred after one million cycles was identified for each material to quantify the degradation in interlaminar shear fracture toughness in fatigue. In addition, residual static toughness tests were conducted on glass/epoxy specimens that had undergone one million cycles without delamination. These residual static tests, and the initial static tests on the tough AS4/PEEK graphite/thermoplastic, exhibited nonlinear load-deflection behavior. For these cases, the load at deviation from nonlinearity was used to determine the interlaminar shear fracture toughness. A linear mixed-mode delamination criterion was used to characterize the static toughness of several composite materials; however, a total G threshold criterion appears to be sufficient for characterizing the fatigue delamination durability of composite materials with a wide range of static toughnesses.