The present study involved the use of light microscopy to investigate the resin-dentin interface, supplemented with examination by scanning electron microscopy. In vivo specimens were processed for (1) scanning electron microscopic examination of acid-rinsed, highly polished, undemineralized surfaces; (2) light microscopic examination of demineralized paraffin serial sections stained with hematoxylin and eosin and according to the Brown and Brenn technique; and (3) scanning electron microscopic examination of dewaxed paraffin sections. The hybrid layer, which was clearly defined with Brown and Brenn stain and confirmed with the acid-rinsed undemineralized specimens, measured up to a depth of 8 jm from the pulp. The resin strings had a rough outer surface and exhibited the appearance of a solid core as they traversed the hybrid layer, becoming gradually hollow within the tubules. This study confirmed the presence of a dentinal hybrid layer within 0.5 mm of the pulp in vivo. The morphology, length, and adaptation of the resin strings were indicative of the degree of wetting with the use of hydrophilic primers.