Bracken (Pteridium species) is an ancient, large and very widespread fern that has long been recognised as poisonous. It contains several toxic compounds including ptaquiloside and thiaminase. Bracken toxicosis manifests as different clinical syndromes in different animals. Cattle develop an acute haemorrhagic syndrome leading to widespread haemorrhage and death. With more chronic ingestion enzootic bovine haematuria occurs, which is characterised by tumours of the upper alimentary tract and urinary bladder. Sheep also develop acute haemorrhage syndrome and cancer, but chronic bracken ingestion causes retinal neuropathy, which can lead to permanent blindness. Bracken toxicosis in monogastric animals such as pigs is because of an induced avitaminosis B1. Diagnosis of bracken toxicosis is based on clinical signs, history of exposure and characteristic findings at postmortem. In cattle and sheep the clinical syndromes are progressive and humane euthanasia is usually necessary in animals with advanced disease. In pigs with bracken toxicosis signs are often non-specific and lead to sudden death, but supplementation with thiamine can be effective.