Three categories of dentinal adhesive are proposed: category I includes adhesives with shear bond strength values between 5 and 7 MPa; category II includes dentinal adhesives with shear bond strengths between 8 and 14 MPa; and category III includes adhesives with shear bond strength values up to 20 MPa. In part I of this article, photomicrographs of the dentinal smear layer and three category I first-generation dentin-adhesive interfaces are presented. The photomicrographs show that the wetting and penetration of the first-generation dentinal adhesives were not adequate to produce high shear bond strengths. When the category I adhesives were tested for shear bond strength, failures occurred at the interface or in the resin adhesive. Future articles will explain wetting and adhesive performance of category II and III adhesives.