The Río Mayo region of northwestern Mexico is a major geographic area whose natural history remains poorly known to outsiders. Lying in a region where desert and tropical, northern and southern, and continental and coastal species converge, it boasts an abundance of flora first documented by Howard Scott Gentry in 1942 in a book now widely regarded as a classic of botanical literature. This new book updates and amends Gentry's Río Mayo Plants. Undertaken with Gentry's support and participation before his death in 1993, it reproduces the original text, which appears here with annotations, and contains information on over 2,800 taxa--more than twice the 1,200 species first described by Gentry. The annotated list of plants includes information on distribution, habitat, appearance, common names, and indigenous uses. A new introduction provides historical background and a review of geography and vegetation. It also describes changes to the land and river wrought by agricultural development, expanded grazing, and lumbering. Throughout the text, the authors have endeavored to provide information on Río Mayo vegetation while emphasizing local knowledge and use of plants, to preserve Gentry's field-oriented focus, and to present botanical information with Gentry's exuberance and style. Río Mayo Plants has long stood as a book that displays a scientist's love of the English language, his fondness for native peoples, and his eye for beauty in nature. This updating of that work fills a gap in the botanical literature of this portion of North America and will be useful not only for botanists but also for biogeographers, taxonomists, land managers, and conservationists.