Purpose: The aim of this study was to evaluate the influence of resin liners on the marginal adaptation to dentin of resin composite restorations photoactivated by a conventional quartz-tungsten halogen lamp (QTH), light emitting diodes (LED), and a plasma arc light (PAC). Materials and Methods: The buccal surfaces of 120 bovine incisors were ground until a flat dentin area was obtained. A circular cavity (4 mm in diameter x 1.5 mm in depth) was prepared in this area. Scotchbond Multi Purpose was applied in the cavities following the manufacturer's instructions. The specimens were randomly assigned into 4 groups according to the lining technique: (1) control (1 adhesive layer); (2) three adhesive layers; (3) Filtek Flow; (4) Protect Liner F. Each group was further divided into 3 subgroups according to the light-curing method: (A) QTH; (B) LED; (C) PAC. All cavities were then restored with a single increment of Filtek Z250 restorative composite and polished. Caries Detector was applied to each specimen for 5 s. Images of dye-stained gaps were observed under a stereomicroscope and transferred to a computer measurement program. Gap values were obtained by dividing the dyed margin length by the total cavity length, then multiplying by 100. Data were analyzed using ANOVA and Tukey's test (p 0.05). Results: All the tested lining techniques significantly decreased marginal gap formation when compared with the control group only for the QTH light groups. There was no significant difference among the light-curing methods. Conclusion: The use of resin liners can effectively reduce the gap formation along dentin margins, but only when QTH light is used Keywords: gab formation, light emitting diodes (LED), quartz tungsten halogen (QTH) lamp, plasma arc (PAC), resin liners