A wide range of lasers have been investigated for their ability to ablate healthy and carious hard dental tissues.7'28 However, before laser ablation of hard dental tissues can become clinically viable, several issues pertaining to safety and efficacy of procedures need to be resolved. Thermal effects of laser irradiation must be quantified, due to pulpal sensitivity to fairly low levels of heat'8 and to thermally-induced structural changes and damage in hard dental tissues. 13,l216 j general, longer pulse durations are less desirable from the thermal aspect, as they produce greater intra-pulpal temperature rises than comparable shorter pulses.27 Thermal studies performed during laser ablation of dentin using the Q-switched nano-second pulsed Nd:YAG laser have demonstrated the capability of this laser for removal of decayed dentin with minimal thermal effects in adjacent dental tissuesJ821 Another issue involves potential deleterious laser-induced alterations in the tooth surface which may, for example, prevent successful bonding of restorative materials to the irradiated surfaces. Such factors include irradiation-induced melting, craterinc, charring or cracking. 22,23,25,26,29,42 Reports of chemical changes in surface composition. 22,3,24,25 Several researchers have investigated the use of lasers for intentional modification of tooth surfaces to improve the tooth surface/restoration interface. In these studies, lasers were usually used solely for surface treatment after conventional cavity preparation. 3334 '35, 3641, 30 31 32,43,44Most of these investigations were performed using fairly long-pulsed or Cw Nd:YAG, C02 or Argon lasers, which would tend to induce significant temperature increases during irradiation, giving rise to concerns about pulpal tolerance of such procedures. Moreover, the results obtained in these studies varied enormously, ranging from laser-enhanced bonding of dentin with composite resins 3 1 to marginal microleakage attributed to the laser tooth surface preparation. 34 33 This wide range of results demonstrates that further investigation and quantification of the issues involved is necessary prior to clinical application of lasers to restorative procedures. Cost/benefit factors should also be considered: dental lasers are currently still fairly expensive devices. Therefore it may be more useful to investigate the tooth/restoration interface after laser use for actual cavity preparation, instead of merely using such a costly device as an adjunct for minor modification of the surface of conventionally prepared cavities. This study was performed to evaluate the tootWrestoration interface created between a composite resin and the tooth surface in teeth where caries removal and cavity preparation were performed either conventionally or using the Q-switched nanosecond pulsed Nd:YAG laser. Microscopic evaluation of the laser-prepared surfaces was performed.