Toxins are produced by numerous microorganisms and invertebrates as well as by higher plants and animals. Venoms are produced by many groups of animals, from coelenterates to vertebrates. While toxins and venoms are the primary toxicological concern in natural ecosystems, they are frequently of importance in agroecosystems and in military deployments. They belong to a very large number of chemical classes and consequently are usually classified according to the groups of organisms producing them, for example, mycotoxins, algal toxins, and insect venoms. Plant toxins are representative of a larger group of phytochemicals known as secondary plant chemicals or plant allelochemicals. They may be acutely toxic, have chronic toxicity, or may be toxic at one dose but therapeutic at a lower dose. Some drugs of abuse are plant toxins abuse. A representative selection of the more important members of each category is provided.