Antinutritional factors in livestock foods may be defined as the substances which either by themselves, or through their metabolic products arising in the system, interfere with food utilization and affect the health and production of animals. These deleterious substances, also referred to as anti-quality factors can be divided tentatively into four groups based on their effects on livestock: (i) affecting protein utilization and depressing digestion (protease inhibitors, tannins, saponins, lectins, etc.), (ii) metal ion scavengers (oxalates, phytates, gossypol pigments, glucosinolates), (Hi) antivitamins, and (iv) those which cannot be put in the above categories (mycotoxins, mimosine, cyanogens, nitrates, alkaloids, photosensitizing agents, isoflavones, etc.). This paper discusses only those deleterious substances which are of prime concern to the farmers and scientists of developing countries. Much emphasis is given to polyphenols compounds, as this class of anti-nutritional factor is the most widespread in non-conventional foodstuffs, i.e. agroindustrial and forestry byproducts. The effective utilization of these non-conventional foods forms the main area of research in the third world countries due to shortages of conventional foods. Efforts have been made to present the current knowledge on their assay methods, effects on animals, mode of action and detoxification/detannification using physical, chemical and biotechnological approaches. Other anti-quality factors viz., mycotoxins, mimosine, cyanogens and nitrates which also cause damage to livestock in developing countries are also discussed briefly.