The physiological and pathological changes associated with occupational exposure to mineral dusts (pneumoconioses) are briefly outlined. The minerals can be in the form of fumes, particulates, or fibres. Excessive exposure to mineral dusts results in scarring of the lung tissue and disturbance of the exchange of gases, leading to respiratory failure; in addition, this makes the lungs more sensitive to infections. The different patterns of scarring as a result of exposure to quartz, coal dust, or asbestos are described. Exposure to asbestos dust is associated with an excess of carcinoma of the lung and tumours of the pleura. There is epidemiological evidence from South Africa and Britain establishing that the pleural tumours (mesotheliomas) occur mainly in those exposed to crocidolite asbestos. In spite of numerous investigations the only factor implicated in the development of these tumours is fibre size. Current studies are concerned with the effects of other fibrous dusts of similar dimensions.