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Objectives:The aim of the current study was to evaluate the resin-dentin micro-tensile bond strength (μ-TBS) using 2 types of universal adhesives with etch-and-rinse and self-etch approaches after 24 hours and 6 months of water storage. Materials and Methods:A total of 20 extracted non-carious human molars were used in this study. Teeth were equally and randomly divided into 2 groups (N= 10 teeth) according to the type of adhesive used; Group I; Adhese® Universal and Group II; Single Bond Universal. Each group was further subdivided into 2 equal subgroups (n=5 teeth) according to the bonding approach used; Subgroup A; using the self-etch (SE) approach and Subgroup B; using the etch-and-rinse (ER) approach. After bonding, each tooth was built up by resin composite, cut into sticks (0.9 mm x 0.9 mm) and stored in distilled water at 37˚C for 24 hours and 6 months. Then, each stick was stressed under tension until failure using a simplified universal testing machine at a crosshead speed of 1 mm/min. Data were statistically analyzed using One-Way ANOVA, Two-Way ANOVA and Tukey HSD Tests. After μ-TBS testing, all debonded surfaces were observed using a stereomicroscope at 50X magnification to determine the modes of failure, which were categorized as adhesive, cohesive or mixed failure. Results:After 24 hours of water storage, there was no significant difference between the μ-TBS exhibited by Adhese Universal using both SE and ER approaches while both approaches differed significantly when using Single Bond Universal (P=0.0003). Furthermore, there was no significant difference when comparing the μ-TBS means of SE groups or ER groups of both adhesive systems. After 6 months of water storage at 37˚C, there was a significant decrease in the μ-TBS values of all groups (p<0.0001) except when Adhese Universal was used with SE approach, there was no difference between the 24 h and the 6 month-groups (p=0.1449). The failure mode analysis was consistent with the μ-TBS test results as the number of adhesive failures increased with decreased bond strength values. Conclusions:When bonding resin-based composite restoratives to dentin, a separate acidetching step is not required when using Adhese Universal, but it is preferred with Single Bond Universal adhesive. Aging markedly contributes to bond degradation of universal adhesives.