The sudden burst of interest in the thymus during the last five years more than justifies the appearance of a monograph on those aspects of its function which bear particularly on immunity and the processes associated with the development of malignant disease. Of those who are actively working in the field I believe that Dr. DONALD METCALF is uniquely suited to produce such a work. His demonstration of a lymphocytosis stimulating factor from thymic tissue in 1956 was almost the first evidence of physiological activity by the thymus. He has played an important part in the analysis of the leukaemogenic process in mice and his recent work on thymic grafting has been almost disconcertingly productive of new ideas. I feel confident that this monograph will be of great value to a wide range of investigators and teachers interested in obtaining a clear summary of the present posi tion. Incidentally, Dr. METCALF has provided a well chosen sample of factual material from which others who may wish to question some of his speculations, can draw their own conclusions. He has been careful to keep the observational and experimental facts clearly distinct from theoretical matters but has not shirked from making his con tribution to the current ferment of immunological theory.