The history of the genus Cleistocactus Lem. (Cactaceae) is reviewed and significant eras outlined. A conservative species concept has been employed to group the over 300 published names into 24 species and 3 heterotypic subspecies. A complete synonymy is given for each of the accepted species along with an expanded description, distribution maps, ecological region associations and a brief commentary. The descriptions are expanded from the respective protologues to include additional data observed both in the field and in ex-situ culture.