While bonding to enamel has been a dependable technique, bonding to dentin is still an overwhelming task--dentin is a naturally soaked organic tissue. The enamel bonding agents of the 1960s and 1970s progressively evolved into complex multibottle or universal adhesives in the early 1990s. These multibottle systems were formulated to bond to enamel, dentin, composite, amalgam, porcelain, and nonprecious metal. More recently, simplified dentin adhesives were introduced following one of two adhesion strategies: total-etch adhesives that include an acid gel to remove the smear layer and dissolve hydroxyapatite (generally 30% to 40% phosphoric acid for 15 to 30 seconds); and self-etching primers that treat the dentin and enamel with a non-rinsing solution of acidic monomers in water without removing the smear layer. Recent attempts to simplify the bonding procedure have led to the introduction of adhesives that do not use a separate acid-etch step and, therefore, do not etch enamel to the same depth as phosphoric acid-based dental adhesives.