The present study reporis on the failure mechanisms at the human dentin-resin interface, with special reference to two specific questions: (1) does failure at the human dentin-resin interface occur by a cohesive or an adhesive mechanism? (2) is the failure mechanism accompanied by a plastic deformation, and if so how important is it? The experimental design, computational analysis, and fractography were applied to two generically different bending systems: Scotchbond-2 (SB2) and Scotchbond-multipurpose (SBM). The theoretical basis of the study required a three-dimensional finite element analysis for calibration of the new geometric factor coefficient. A minimum geometric factor coefficient (Ym) of 17.3 was then obtained which was vital for the determination of the interfacial fracture toughness. Accordingly, the dentin-resin interfacial fracture toughness (GIC), for the SB2 and for the SBM were 30.22 ± 5.61 and 49.56 ± 7.65 J m-2, respectively, which were significantly different (p < 0.01). Both SB2 and SBM interfaces with dentin displayed significant degrees of plasticity (0.15 and 0.19) which were beneficial to crack resistance. Thus, correcting for the plasticity, the GIC for SB2 and for SBM increased to 42.83 ± 7.75 and 74.97 ± 10.47 J m-2, respectively. The fractography of the two systems reflected these numeric differences. SB2 showed largely interfacial adhesive failure, while SBM showed adhesive-cohesive failure with occasional dentin adhesions attached to the composite interface and vice versa.