STUDIES have been made of the histopathology of the caries lesion in normal, hypocalcified and hypoplastic teeth, particularly in the early stages. In this paper an attempt is made to re-orientate the interpretation of these stages with special reference to the bacterial invasion phase of caries. With regard to the initial demineralization phase, I think we can, within the limits of the microscope, almost prove it to be of environmental origin. It is proposed to refer briefly to some observations on the bacterial plaque and to outline certain histological appearances of the destruction of the enamel, dentine and cementum and to present some correlations between these appearances and those in two cases in which bacterial penetration of enamel in one case, and enamel, dentine and bone in the other, occurred under conditions different from those usually associated with dental caries. Human teeth only have been used. The various methods of preparation including the decalcification techniques used for the recovery of the organic matrix of both normal and hypocalcified enamel are, with certain modifications, those described in a previous publication (Hodson, 1952).