The structure of fibrous filters has been analysed by the examination of sections of embedded filters, An experimental complication is that the open structure of the filters, which is necessary for efficient filtration, results in a lack of rigidity, This requires the fibres to be fixed with adhesive vapour, conferring rigidity at the points of fibre-fibre contact, before the filters are embedded. Low-viscosity epoxy resin is used as the embedding medium, and sections are produced by cutting, followed by polishing with fine abrasive. Sections of a number of filters have been photographed, and those of a lightly needled carded filter have been analysed with the use of a Seescan Image Analysis system. The parameters that have been studied are: the packing fraction, the fibre diameter distribution, the distribution of fibre alignment, and a measure of the fibre arrangement, Fibres tend to align in the plane of the material, but there is no strongly preferred direction within this plane, The fibre arrangement has been found to be slightly more ＇clumped＇ than a random arrangement.