Introduction: Resin cements are used for cementation of indirect restorations. A high-quality adhesion to tooth structure is primordial for the success of indirect cemented restorations. Objectives: Were to assess the shear bond strength of RelyX ultimate adhesive resin cement to both enamel and dentin when used in different application modes (self-etch and total-etch) and to study resin-tooth interfaces using SEM. Materials and methods: 40 enamel specimens and 40 dentin specimens were used in this study. Each substrate specimens were randomly divided into four groups of 10 specimens each according to resin cement applied: Enamel groups: Group A1: RelyX ultimate as total-etch; Group A2: RelyX ultimate as self-etch; Group A3: Variolink II; Group A4: Multilink automix. Dentin groups: Group B1: RelyX ultimate as total-etch; Group B2: RelyX ultimate as self-etch; Group B3: Variolink II; Group B4: Multilink automix. Resin cements were built over the surfaces following manufacturer's instructions in a plastic tube. All specimens were thermocycled (500 cycles, 5°/55°C, 15 sec dwell time) then shear bond strength was measured in MPa and data were analyzed using F-test (ANOVA) and Post Hoc test. Three specimens from each group were further sectioned, gold sputtered and evaluated under SEM. Results: All enamel groups showed a statistically significant difference for shear bond strength with P <0.001. Group A1 revealed the highest mean value of 35.60 MPa. While for dentin, there was no statistically significant differences among groups with P=0.053. Interfacial morphology of RelyX ultimate in total-etch mode showed better interface morphology and improved tag length compared to respective self-etch counterpart Conclusions: An etching step prior to multi-mode adhesive resin cement increased its bond strength values and improved its penetration pattern to both enamel and dentin, also length of resin tags formed by total-etch adhesives seems not to play an important role in determining bond strength

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