Objective: The purpose of this study was to evaluate the effects of glass-ceramic inserts and different application techniques of resin composites on marginal leakage. Method and materials: Three groups of 10 standardized, round, Class V cavities were prepared on the buccal surface of 30 extracted human molar teeth. Gruop 1 received resin composite restorations, placed in one increment (bulk). Group 2 received resin composite restorations that were placed in two increments, and gruop 3 received resin composite restorations with beta-quartz ceramic inserts. After the teeth were thermocycled and placed in 0.5% basic fuchsin for 24 hours, they were sectioned and examined for microleakage. Results: On the occlusal margins, there was no statistically significant difference between the restorations placed with the incremental technique and the restorations with beta-quartz inserts. The restorations placed with the bulk technique had significantly more microleakage than did the others. On the gingival margins, the restorations with beta-quartz inserts exhibited significantly less microleakage than did the other groups. The restorations placed with the bulk technique showed significantly more dye penetration than did the others. Conclusion: The use of a glass-ceramic insert reduced the marginal leakage of Class V resin composite restorations.