Pleural plaques are ubiquitous in asbestos exposed populations, but their pathogenesis remains obscure. Fibre burden studies indicate that plaques appear at fibre levels much below those seen in subjects with asbestosis, but considerably above those found in the general population, and that they appear to be associated with high aspect ratio fibres in the parenchyma. Analysis of parietal pleura and plaques typically reveals short fibres of chrysotile,although one recent study has also found long amphibole fibres in localised areas of the parietal pleura. Plaques are seen in subjects exposed to all types of asbestos, but it is unclear whether chrysotile actually plays a role in their pathogenesis, whereas amphiboles clearly produce plaques. Plaques probably(but not certainly) represent a direct effect of fibres reaching the pleura, but there is no understanding of what factors initiate plaque formation, and why they are completely different in structure and distribution from asbestosinduced diffuse pleural fibrosis.