Dietary fibre is classically described as plant material that resists digestion by human digestive enzymes. It is a generic term that includes a number of substances of unique chemical structure, characteristic physical properties, and individual physiological effects. With the exception of lignin, all of the materials that are classified as dietary fibre are fermented to some extent by the colonic bacteria. The products of this fermentation are hydrogen, methane, carbon dioxide and short-chain volatile fatty acids such as acetic, propionic, butyric and valeric. The major constituents of dietary fibre are the principal components of the plant cell wall: cellulose, hemicellulose, pectic substances and lignins. However, the term dietary fibre has been expanded to include natural or semi-synthetic polysaccharides such as gums, mucillages, algal polysaccharides, chemically modified cellulose and resistant starches. The major carbohydrates of saccharidic fibres are glucose, mannose, galactose, xylose, arabinose, rhamnose, fucose, galacturonic and glucuronic acid and in some cases (i.e. carrageenans) osidic sulphate esters. Lignins are complex, cross-linked phenylpropane polymers containing sinapyl, coniferyl and coumaryl alcohols. The major reasons for having fibre (in the broad sense of the word) in food are either nutritional (low calorie bulking agent--health promoting effect), technological or both. Its concentration in food may vary from a fraction (gums, mucillages, algal polysaccharides) to a few percent, with a maximum near 10% (plant polysaccharides). Until recently most dietary fibres were consumed as a result of their natural occurrence in fruits, cereals or vegetables and only a few of them were added to food, mainly for technological reasons (for example gums or algal polysaccharides like carrageenans or pectins). More recently, because of the development of food industry, and because of their recognized role in human health and preventive medicine, a great variety of dietary fibres has been developed to be added to marketed food products (presently some 80 different products are used in US food products). For the same reasons, commercial use of dietary fibre is likely to increase in the near future, together with research for new and well characterized products with specific physiological or technological properties. Moreover, very soon biotechnologically synthesized and/or processed dietary polysaccharides with fibrelike activities will become available.