OBJECTIVES: The porcelain veneer technique bonds a thin porcelain shell to a minimally prepared tooth surface (enamel and/or dentin) with a luting composite in combination with an adhesive system. When complete, two adhesive interfaces are formed--resin to etched porcelain and resin to tooth structure. The purpose of this laboratory investigation is to analyze the ultra-structure of these two adhesive interfaces in order to find an explanation for some clinical phenomena and/or shortcomings of bonded porcelain veneers. METHODS: The surfaces of four porcelain disks were treated successively by sandblasting, etching with hydrofluoric acid, ultrasonic cleaning, and silanizing. The effect of each step on the surface structure of the porcelain was evaluated by field-emission scanning electron microscopy (FE-SEM). In addition, the effect of acid etching the tooth surface of veneer preparations on seven extracted upper anterior teeth was similarly determined. Finally, pre-treated porcelain veneers were luted to veneer preparations on 12 extracted upper anterior teeth by means of a contemporary total-etch adhesive system and a photo-polymerizable luting composite. The tooth/luting composite and the luting composite/porcelain interface of the veneer complex were then evaluated ultra-morphologically by FE-SEM after the specimens had been etched with an argon-ion beam to enhance the surface relief. RESULTS: FE-SEM imaging of the tooth/luting composite/porcelain interface showed strong micro-mechanical interlocking of the luting composite in the micro-retentive pits both of the acid-etched tooth surface and in the etch pits of the acid-etched porcelain surface. From this ultra-morphological perspective, the etched porcelain surface is more retentive than the etched tooth surface. At the tooth surface, cervical aprismatic enamel and exposed dentin showed the least resin-receptive surface texture. However, in these areas, no separation of the interface was observed when a multi-step total-etch adhesive system was used. SIGNIFICANCE: The strong interlocking of the luting composite into the retentive etch pits of both the porcelain and tooth substrates contributes to strong adhesion of porcelain veneers with good retention. This study suggests that a modern multi-step total-etch adhesive system can produce adhesion even to the less retentive cervical enamel and exposed dentin. Further clinical research is necessary to evaluate the long-term retention of porcelain veneers and to correlate these in vitro observations with clinical performance.