Technical Abstract: When alternative forages are available, grazing animals generally avoid eating poisonous plants. However, when toxic plants are included in prepared feed and hay, they are often consumed and they can poison animals. Alternatively, some normally safe forages can under certain conditions, produce and accumulate toxins and poison livestock. As poisoning often results in fatal or costly debilitating disease, identifying toxic plants and their toxins in prepared forages is essential. The purpose of this paper is to review basic principles of identifying contaminated feeds, review how to collect and where to have samples analyzed, and briefly introduce several toxins and poisonous plants that are likely to contaminate feed in the western United States.