Purposes: This study aimed at evaluation of two different commercially available calcium silicate materials (Biodentineand mineral trioxide aggregate [MTA] Plus) used as dentin substitute.  Materials and Methods: Sixty Class II cavities were prepared in extracted mandibular third molars, with margins extending 1mm below the cementum–enamel junction.The samples were divided into three groups on the basis of dentin substitute used: resin modified glass ionomer cement, Biodentine, and MTA Plus. Cavities were restored with composite resins in an “open sandwich” technique.The samples were subjected to alternate aging in phosphate buffered saline and cyclic loading. Marginal adaptation was evaluated in terms of “continuous margin” at the gingival margin, using a low vacuum scanning electron microscope. Statistical analysis was done with two-way analysis of variance with Holm-Sidak’s correction for multiple comparisons.  Results: The glass ionomer group and Biodentine group presented an overall 83% and 91% of continuous margins, with no difference between them. MTA Plus showed least values of continuous margins. Granular deposits were seen over the surface of Biodentine and MTA Plus.  Conclusions: Biodentine and resin-modified glass ionomer cement, when used as a dentin substitute under composite restorations in open sandwich technique, gave satisfactory marginal adaptation values. (J Esthet Restor Dent. 2015 May-Jun;27(3):167-75. )