Seventy-five- to one-hundred-micron slices of undemineralized human teeth were examined by transmitted light, incident light and polarized light microscopy, to determine the nature of Tomes' granular layer in root dentine. Transmitted light examination showed that the granules occupied differing levels within the section. Incident light microscopy showed that the granules were true spaces and polarized light microscopy showed that fibre orientation was not the cause of the granular layer. When thin sections (1 μ-0.75μ) of a tooth slice containing Tomes' granular layer were examined by light and electron microscopy no evidence of any spaces were found in the region where they had been demonstrated in the thick 75-100 μ section. Looping of the terminal portions of the dentinal tubules was found. A model system has been used to show that sectioning of this loop region could produce a space visible in thick sections and corresponding to a granule of Tomes' layer. It is suggested that this looping of dentinal tubules in root dentine, and not in the dentine beneath enamel, is the result of different rates of dentine deposition in these two areas.