Objective: To compare the ability of four light-emitting diode (LED) curing lights to polymerize five composite resins in 10 seconds at 4 and 8 mm. Method and Materials: Two second-generation, single-peak LED curing lights (Bluephase 16i, Ivoclar Vivadent, and LEDemetron II, Kerr) and two third-generation polywave LED curing lights (UltraLume 5, Ultradent, and Bluephase G2, Ivoclar Vivadent) were compared. Three examples of each brand of curing light were used, and their light outputs were measured with a spectro-radiometer. Five composite resins (Filtek Supreme A2B, 3M ESPE; Vit-l-escence A2, Ultradent; Aelite LS Posterior A2, Bisco; and Tetric EvoCeram A2 and Tetric EvoCeram Bleach M, both Ivoclar Vivadent) were polymerized for 10 seconds at 4 and 8 mm from the end of the light guide. The Knoop microhardness (KHN50gf) was measured at 49 locations across the top and bottom surfaces of the specimens to determine the ability of each light to cure each brand of composite in 10 seconds. Results: At 4 and 8 mm, the Bluephase G2 light delivered the broadest spectral range of wavelengths, greatest irradiance, and energy density. The Bluephase G2 always produced harder, better-cured resins compared to the other three lights. Overall, the ability of the lights to cure these five composites was ranked from highest to lowest: Bluephase G2, UltraLume 5, Bluephase 16i, and LEDemetron II (ANOVA with REGWQ multiple comparison test, P .01). Conclusion: This study suggests that polywave LED curing lights should be used in preference to single-peak LED curing lights. Keywords: chemistry, composite resins, curing light, dental materials, hardness testing, Knoop microhardness, light-emitting diode, physical properties, radiation effects