Objectives: Due to their superior conversion rate as well as to their optimum spectral emission LED’S are most likely to shape the next generation of curing lights. It would therefore be favourable to further investigate and compare them with the standard curing methods. The purpose of this study was to investigate the effect of a LED’s on microleakage of a dental composite. Materials and Method: 64 Class V cavities were prepared in human premolars, conditioned with 35% phosphoric acid and treated with the bonding agent Prime & Bond NT. They were randomly divided into four groups of 16 each and incrementally filled with the composite Esthet-X. The first group was cured by means of a Spectrum 800 set at 400W/cm2 for 20 seconds per increment, the second group with the Freelight operating in standard mode, the third group with the same light in soft start mode and the fourth group with an Ultralume-2. The specimens were thermo-cycled in a 0.5% basic fuchsine solution, sectioned and evaluated for dye penetration using a scoring system of 0-4.The number of restorations exhibiting each degree of leakage was recorded per light source and summarised in a 2-way contingency table of observed frequencies. To test for significant differences between the lights, a Chi-square test was done (p<0.01). Results: A cross tabulation of the microleakage scores for dentine revealed that except for the Freelight in soft start mode, where more 3’s occurred, the lights predominantly had scores of 4. For the enamel side there were no significant differences between the lights with all of them scoring predominantly 0’s. Conclusion: Although LED’s offer numerous advantages to the clinician, they have not proven their superiority in reducing microleakage over the halogen LCU's, and it is still viable to use a properly maintained halogen light.