The ultrastructural nature of the interface between dental hard tissue and soft and calcified deposits was studied in undecalcified and decalcified thin sections from 28 extracted human teeth. Uncalcified plaques contained clusters of densely packed microorganisms which adhered to cuticular structures or directly to the tooth surface. In calcified deposits, on the other hand, larger amounts of intercellular material generally separated the cells from each other and from the tooth surface. An intimate relationship existed in most instances between the intercellular matrix of calculus and the organic matrix of the dental hard tissue as well as between the mineral crystals of calculus and those of the underlying tooth surfaces.Mechanical locking of the deposits in cracks, resorptions, carious lesions, and other defects of the tooth surface was frequently observed. Moreover, calculus was closely adapted to the sub‐microscopic irregularities which were present even on “smooth” tooth surfaces. At the ultra‐structural level, the predominant mode of attachment appeared to be the adhesion of organic interbacterial substance to the tooth surface, in addition to intercrystalline forces of inorganic nature acting between crystals of calculus and underlying tooth substance. The relative importance of the latter factor presumably increases as the concrement