©Operative Dentistry, 2021.OBJECTIVE: To compare the effects of conventional (hand-placed), sonic, or preheated insertion techniques on the internal adaptation of bulk-fill resin composites. METHODS AND MATERIALS: A total of 150 freshly extracted human third molars were used to prepare standardized cylindrical occlusal cavities. Teeth were divided into five main groups according to the resin composites: 1 incremental (Clearfil Majesty Posterior [CMP]) and four paste-like bulk-fill (SonicFill 2 [SF2], VisCalor bulk [VCB], Filtek One bulk-fill restorative [FBR], and Tetric EvoCeram bulk-fill [TEB]). Each main group was divided into three subgroups according to the placement technique: conventional, preheating, and sonic delivery (n=10). In the conventional placement technique, cavities were filled manually. In the sonic insertion technique, a specific handpiece (SonicFill Handpiece; Kerr Corporation) was used. In the preheating technique, a heating device (Caps Warmer, Voco, Cuxhaven, Germany) was used to warm the resin composites before placement. Internal voids (%) of the completed restorations were calculated with microcomputed tomography. Data was analyzed with two-way analysis of variacne followed by Tukey's multiple comparisons test (α=0.05). RESULTS: All resin composites showed fewer internal gaps with preheating compared with the conventional placement (p<0.05). For all resin composites other than SF2, preheating provided fewer internal gaps than that of the sonic placement (p<0.05). Sonic placement led to fewer internal gaps compared with the conventional placement, but only for SF2 and FBR (p<0.05). For the conventional placement, the lowest gap percentage was observed with the incremental resin composite (CMP, p<0.05). Among all groups, the lowest gap percentages were observed for preheated VCB followed by sonically inserted SF2 (p<0.05). CONCLUSION: The best internal adaptation was observed in sonically inserted SF2 and preheated VCB, which were the manufacturers' recommended insertion techniques. Preheating considerably improved the internal adaptation of all resin composites, except for that of SF2.