Objective: To assess microtensile bond strength (MTBS) of bulk-fill resin-composites in high configuration-factor cavity preparations. Methods: Non-carious 48 sound human molars were selected and divided into two equal groups. Occlusal surfaces of the first group were ground flat to expose superficial dentin below the occlusal surface. Flat 2.5-mm thick coronal slabs were obtained from the second group and cemented to occlusal surfaces of first group. Teeth were prepared (5mm mesio-distal x4mm bucco-lingual x4mm deep), randomly divided into two main equal groups, according to the adhesive system utilized (etch-and-rinse and self-etch technique), and further subdivided into 6 equal subgroups according to restorative protocol (3 per main group). First subgroup was restored with bulk-fill flowable composite and an increment of resin composite. Second subgroup was restored with a single increment of bulk-fill composite, while third subgroup was restored incrementally using resin composite restorative material. Bonding and restoration were done according to manufacturer instructions followed by serial sectioning into 0.9x0.9mm thick beams (n=16/subgroup). Samples were subjected to MTBS test (0.5 mm/min), and statistically analyzed (ANOVA, Tukey’s post-hoc, P≤0.05). Failure modes were examined (digital microscope and field emission scanning electron microscope). Results: Etch-and-rinse group recorded statistically significant higher MTBS mean value than self-etch group treated after 24-hour aging (p<0.05), incrementally-applied conventional composite recorded statistically significant lower MTBS mean value than bulk-fill composites (p< 0.05), and bulk-fill flowable composite recorded highest MTBS mean value among all groups (p<0.05). Conclusion: 1. Bulk-fill flowable composite based on SDR technology and Quixfil packable bulk-fill composite provide a satisfying solution to the problematic restoration of high configuration-factor and deep cavity preparations. 2. The adhesive protocol had a major influence on the MTBS in high configuration-factor cavity preparations. Two-step, etch-and-rinse adhesive systems are more reliable than one-step self-etch adhesives in bonding to such cavity designs