Mycotoxins are secondary metabolites produced by fungi, which may be present on a variety ofcrops. They are considered a major issue worldwide because of their harmful effects on animals.These contaminants lead to great economic losses, especially in pig and poultry husbandry. Over400 mycotoxins have been identifi ed. However, only few of them have a signifi cant toxic effectand are of major concern. In this paper, the most important mycotoxins are described, includingdeoxynivalenol (DON), T-2 toxin (T-2), zearalenone (ZON), fumonisin B1 (FB1), ochratoxinA (OTA) and afl atoxin B1 (AFB1). For each toxin, its chemical structure, mode of action andsymptoms of acute and chronic toxicity in pigs and poultry are discussed.