Bracken (Pteridium spp.) is a ubiquitous fern which has been described as one of the five most common plants on the earth. The toxic effects of bracken on livestock have been recorded since the end of the 19th century, and extensive and intensive investigations for the bracken toxin(s) led to the isolation of ptaquiloside in 1983 as the major, but unstable, toxin of bracken. This review concentrates mainly on the results of the scientific investigations into ptaquiloside, and cites 133 references.