Smear layers are created on hard tissues whenever they are cut with hand or rotary instruments. This thin (1-2 microns) layer of denatured cutting debris is very tenacious and, in fact, is often the surface to which restorative materials are luted. The solubility characteristics, chemical reactivity and the structure-function relationships of this layer have not yet been well-defined. During creation of the smear layer, cutting debris is forced variable distances into dentinal tubules. These so-called smear plugs, together with the smear layer decrease dentin permeability, dentin sensitivity and surface wetness. Bonding adhesive resins to smear layers appears to limit the theoretical bond strength unless the smear layers are loosened or partially removed. Future research in this area will include the use of surface analytical techniques such as Auger electron spectroscopy and ESCA. These techniques are required because of the thinness of the smear layer. It is clear that the nature of this critical interface between dental materials and cut hard tissues remains largely unknown. This field will provide fertile ground for future research.