In an attempt to clarify the nature of the human cemento-dentinal junction, ground sections of incompletely formed and fully formed extracted teeth were prepared and their histology compared with their microradiographic appearances. The results showed that incompletely formed teeth possess distinctive surface layers outside the granular layer of Tomes. The evidence indicates that these layers are of dentinal origin; their presence during development supports previous explanations by the author of the hyaline layer of Hopewell-Smith and of so-called intermediate cementum. The results also indicate that the granular layer of Tomes does not represent the outer limit of root dentine. The relationship of these surface layers to the definitive cementum which is present in fully formed teeth was studied in both young and older patients. From the results it was concluded that cementum formation begins in the more apical region of the teeth at a time when root formation is well advanced, and that it spreads towards the crown rather than in the generally accepted reverse direction.