Clinical Relevance The marginal gap of ceramic onlays manufactured by Cerec 3D present a similar marginal gap whether the optical impression is taken intraorally using the Cerec powder or extraorally using a stone model. SUMMARY Objectives: This study evaluated the marginal gaps on several surfaces of onlays created with the Cerec 3D system using one intraoral and two extraoral optical impression methods. Methods: A human molar (#19) was mounted with its adjacent teeth on a typodont (Frasaco) and prepared for a MODL onlay. The typodont was assembled in the mannequin head in order to simulate clinical conditions. The same operator took 36 individual optical impressions using a CEREC 3D camera. For group 1 (IP), a thin layer of titanium dioxide powder (CEREC powder-VITA) was applied directly onto the surface of the preparation for imaging (n=12). For group 2 (EP), a sectional impression was taken with hydrocolloid Identic Syringable (Dux Dental), a die made with polyvinylsiloxane KwikkModel Scan (R-dental Dentalerzeugnisse GmbH) and powdered with titanium dioxide for imaging (n=12). For group 3 (ES), a sectional impression was taken with PVS and a sectional stock tray, a die fabricated in stone (Diamond die- HI-TEC Dental Products) and the die being imaged without powdering (n=12). One operator designed and machined the onlays in VitaBlocks Mark II for Cerec (VITA) using a CEREC 3D. The marginal gaps (μm) were measured with an optical microscope (50×) at 12 points, three on each surface of the MODL. The results were analyzed by two-way ANOVA/Tukey's ( p =0.05). Results: The overall mean marginal gaps (μm) for the three methods were: IP=111.6 (± 34.0); EP=161.4 (± 37.6) and ES=116.8 (± 42.3). IP and ES were equal, but both were significantly less than EP. The pooled mean marginal gaps (μm) for the occlusal = 110.5 (± 39) and lingual = 111.5 (± 30.5) surfaces were equivalent and significantly less than the distal = 136.5 (± 42.5) and mesial = 161.1 (± 43.3). Conclusion: The marginal gap of CEREC 3D onlay restorations was not different when the optical impression was taken intraorally vs extraorally using a stone cast that does not require powdering. The lingual and occlusal surfaces showed the lowest gaps.