Alsoofi AA,¹El-Shehawy TM,² Zaghloul NM^{3 1} Postgraduate student in Operative Dentistry Department, Faculty of Dentistry, Mansoura University ² Lecturer, Operative Dentistry Department, Faculty of Dentistry, Mansoura University. ³ Associate Professor of Operative Dentistry Department, Faculty of Dentistry, Mansoura University. Abstract: Objectives:This study was conducted to evaluatein-vitro shear bond strength of resin composite to two different dentin substitutes: resin-modified glass-ionomer cement and Biodentine, using a universal adhesive in etch-and-rinse and self-etch adhesive modes. Materials and Methods:Eighty discs of two different dentin substitutes:  RMGIC (n=40) and Biodentine ( n=40) were constructed in a split Teflon mold (4mm diameter and 2mm thickness) according to the manufacturer's instructions. These 80 samples were divided into two groups: Group A (RMGIC) and Group B (Biodentine).  Using the same universal adhesive,  each group was then further subdivided  equally into two subgroups: The first one was bonded with E&R adhesive mode and the other one was bonded with SE adhesive mode with another Teflon mold (  4mm diameter and 6mm thickness). Shear bond strength  analysis was performed at a cross-head speed of 1mm/min.  Debonded specimens were examined under the  stereomicroscope at 20× magnification to evaluate the failure modes. Results:high statistically significant difference was observed between the two dentin substitutes (P<0.001**): the shear bond strength of resin composite to RMGIC was significantly higher than that with Biodentine.   It was Also found that there was no significant difference between the etch-and-rinse and self-etch adhesive modes, when were applied either with RMGIC or Biodentine.Modes of failure were predominately mixed failure modes in RMGIC specimens while in Biodentine specimens were predominately cohesive failure modes within Biodentine substrate. Conclusions: Shear bond strength of resin composite to RMGIC was significantly higher than that to Biodentine..