Objectives: To determine the effect of proper light-curing instruction on the radiant exposure (energy density) delivered by dentists using six dental curing lights to a posterior Class 1 restoration. Method and Materials: Twenty-five dentists attending a professional meeting were instructed to position a patient simulator (MARC-PS, BlueLight), as they would for a patient, and then to expose the simulated Class 1 maxillary second molar preparation for a specified amount of time. At this point, the dentists were unaware of the purpose of the experiment. Each participant used three different curing lights, and the irradiance and radiant exposure (J/cm2) delivered to the preparation was recorded. Participants were then informed of the purpose of the exercise, and given specific light-curing instructions and training using the patient simulator, after which they re-exposed the same preparation using the same curing lights. Pre- and post-instruction radiant exposure values were compared using one-way ANOVA (α = .05), and for each light among all operators using a two-tailed, paired Student's t test. Results: There was a wide variation in the radiant exposure delivered by the dentists and by the six curing lights, from 2.9 to 15.4 J/cm2. Before receiving additional light-curing instruction, 68% of dentists delivered less than 10 J/cm2. The radiant exposure delivered increased significantly (P .001) by up to 30%, as a result of training using MARC-PS. Conclusion: The results indicate that some of the dentists participating in the present study delivered an inadequate amount of radiant exposure before instruction. More energy was delivered after a short training session using the MARC-PS. Reinforcing the proper photo-curing techniques may improve the outcome when placing resin-based restorations. Keywords: clinical performance, composite resin, light curing, operative dentistry, polymerization