The quest ion of the aetiology of silicosis is dicussed on the basis of the available results from statistical investigations in the pneumoconiosis fre- quency in coal-mining, from lung dust analyses, and from animal experi- ments. The data achieved from lung dust analyses in many countries and from anima I investigations with artificial, quartz-containing dusts as weH as original dusts out of coHieries or isolated from human lungs are in favour of the opinion that free silicic acid in breathable particle size is aprerequisit for the origination of silicosis in coal-miners. There are other circumstances, e. g. bacterial infections, which may decisively influence the development of silicosis. In view of the pathogenesis of silicosis the present research is concentrated on those primary reactions between dust and ceH which cause the damaging of the macrophage. Experiments in vitro and in vivo have, to some extent, elucidated the cytotoxic effects during and after the phagocytosis of dust.