Quinolizidine and piperidine alkaloid teratogens from Lupinus, Conium, and Nicotiana genera have been identified as causes of birth defects in livestock induced by poisonous plants. Many defects now known to be related to poisonous plant ingestion were once thought to have a genetic origin. This supposition delayed diagnosis, reporting, and understanding of such birth defects, because breeders and producers feared the news would make it difficult to sell breeding stock. Defects caused by quinolizidine and piperidine teratogens include cleft palate and contracture-type skeletal defects such as arthrogryposis, scoliosis, torticollis, and kyphosis. Teratogens have been identified, differences in susceptibility to teratogenic compounds among livestock species have been elucidated, periods of gestation when specific types of birth defects occur have been determined, and information about mechanism of action has been developed.