To compare the proximal adaptation and the marginal seal of different types of posterior composite resin restorations, 48 human molars were obturated in vitro with two direct techniques (three-sited light-curing technique and a multilaver technique) and one indirect technique. As a control, some specimens were obturated with amalgam. Half of the specimens were subjected to thermocycling. The micromorphologic adaptaion was evaluated with the scanning electron microscope and microleakage was evaluated with a dye infiltration test. Depending on the techniques used and on the level of the proximal gingival limit, only one half to two thirds of the investigated margins were rated excellent. The infiltration test revealed less leakage in enamel than in cementum. However, the results did not show statistical dlfferences in adaptation or in seal among the three composite resin techniques tested. Adaptation and seal in composite resin restorations were found to be inferior in all aspects to those of the amalgani restorations. The thermocycling did not induce marked changes in the results.