A description of chemical agents for protecting plants against their pests would be incomplete without mentioning phytoalexins. These are substances with bactericidal and fungicidal effects. They are produced when the plant’s integrity is threatened. These compounds are synthesized by the plant immediately after infection by microorganisms such as bacteria or fungi, in order to inhibit their spread, growth, or reproduction within the plant. The chemical nature of many of these substances has now been elucidated. It has been found that they belong to various chemical classes and groups. Most of them contain alcoholic or phenolic groups, which allow them to bind to carbohydrate residues in the form of glycosides. The glycoside form of phytoalexins is inactive; in healthy plant tissue, they are “dormant.” When the tissue is damaged, specific enzymes (hydrolases) are activated, releasing the phytoalexin from its carbohydrate carrier. As a result, the attacking microbes or fungal hyphae are killed. By attacking the plant, the pathogen thus triggers the very trap that nature has “set” for it. Phytoalexins represent the plant’s second line of defense. The first are the phytoncides, which combat the pest from a distance. If they fail to stop it, the phytoalexins complete the task.