The traditional principles of operative dentistry have been challenged since Dr. Buonocore introduced the aid-etch technique in 1955. In spite of the numerous changes in clinical protocols and adhesive techniques, adhesion to dentin remains difficult. The importance of micromechanical bonding to dentin has been recognized over the last decade. Researchers now believe that dentin adhesion relies primarily on the penetration of adhesive monomers into the filigree of collagen fibers left exposed by acid etching. Two main strategies are currently in use for bonding to enamel and dentin: the self-etch technique and the total-etch technique. The efficiency of either bonding strategy depends very much on the dentin substrate used for bonding. Laboratory tests use ideal dentin from extracted teeth, which does not reflect the clinical reality. Clinical studies are the ultimate test for any dental adhesive material.