Polygonum is a taxonomically difficult, highly variable genus. Plants of Polygonum are annual or perennial herbs, shrubs, or vines of moist habitats, many becoming tenancious weeds in disturbed areas. Section Echinocaulon, usually defined by the occurrence of recurved prickles on stems, petioles, and abaxial surfaces of major leaf veins, consists of 21 species. The taxonomic treatment of Echinocaulon in this volume is based on comparative morphology, biogeography, cladistic analyses, and flavonoid chemistry. Includes keys, descriptions, and distribution maps.