Technical Abstract: Horses are relatively selective grazers and generally they are poisoned less frequently than other livestock. However, there are exceptions. Some poisonous plants are palatable to horses and exposed horses readily eat them. Other plants may be eaten by some horses even though they are unpalatable to the rest of the herd. As individual horses may actively seek and eat toxic plants, it has been suggested they become addicted to certain toxic plants. There is little experimental support for addiction, but individual horses do develop strong feed preferences. These horses certainly pose greater risk to poisoning. As a rule, both preference and palatability are different when poisonous plants are dried and included in stored feed. Most horses readily accept toxic plants that are included in hay or processed feed. In herd situations competition for food can enhance poisoning as animals hurry to eat what they can or fight to keep lesser status animals from eating. As antidotes are rare, plant toxin-induced diseases can be irreversible and some poisonings lethal. Knowing the risks and sequelae of poisoning is essential. The purpose of this series of papers is to introduce hepatotoxic plants that poison horses in North American, describe poisoning and the subsequent plant-induced disease and outline current recommendations for treatment as well as management practices to avoid exposure to the plants.