The clinical and histological manifestations of leprosy are known to be closely correlated with the immunological state of the patient, which determines prognosis and constitutes the natural basis for the classification of this disease. A classification based on this correlation has come to be widely used but needs to be brought up to date and expanded in the light of more recent experience. The author of the present paper presents a much fuller account of the histological side of the classification, taking into account the results of recent experience. This histological classification has been found to provide a workable and widely applicable system, different histologists achieving remarkably good agreement with one another.