Notes are given on the origin, taxonomy, biology, identification, utilization, distribution and spread of cockleburs, particularly Xanthium spinosum and X. strumarium. X. strumarium is prevalent in the USA, particularly in the E. Xanthium spp. may set seed without fertilization of the ovule, resulting in local populations in which the plants are all genetically equivalent but which differ in minor characteristics from other populations. Toxicity is similar among all the Xanthium spp. Only the seedling stage is poisonous when the seeds contain hydroquinone. Plants at the cotyledon stage are toxic as well as palatable to livestock. Toxicity decreases rapidly as the first true-leaves develop, but it is not lost on drying. Xanthium spp. are sometimes found growing with Arctium spp., another European immigrant weed genus common in the USA.