Darnel (Lolium temulentum L.), the subject of this review, is botanically and culturally significant because of its evolutionary origin as a mimic weed of cereals and its reputation as a source of potent psychoactive toxins. Evidence from molecular phylogeny, palaeontology and archaeology allows the source and spread of darnel in time and space to be reconstructed. Contemporaneously with the progenitors of wheat and barley, at the dawn of agriculture in the Fertile Crescent region of Mesopotamia and the Eastern Mediterranean, darnel was derived from a perennial ancestor that was subject to the same human-mediated selection pressures as the earliest cereal species, and shares with them the domestication traits of annuality, self-fertility, high harvest index and non-shattering grains. Because it combines the characteristics of cereals with those of forage species of the Lolium-Festuca complex, L. temulentum is a useful experimental subject for the study of the physiology of temperate grasses. In particular, it has been a model for research on the control of flowering by day length, as well as investigations of vegetative development, resource allocation and responses to abiotic stresses. Recent studies of the chemical basis for darnel's noxious reputation reveal a complex picture in which endophytic fungi, nematodes and pathogenic bacteria separately or in combination account for the toxicity of the darnel grain. The relationship, and frequent historical conflation, of darnel and ergot poisoning is considered in detail. Finally, some examples of literary allusions to L. temulentum, from Aristophanes to Shakespeare, are given.