The genus ''Gymnocalycium'' has been popular with cactus fanciers for many years. It is widespread in the wild, with most species occurring in northern Argentina and southern Bolivia, and a handful in southern Brazil, south-western Paraguay and Uruguay. New species or variants of species have been discovered steadily since the recognition by Britton and Rose in 1922 of Karl Pfeiffer's genus ''Gymnocalycium'', proposed by him some 70 years before. Some of the earliest of cactus illustrations are of species of ''Gymnocalycium'', then called under the umbrella names of ribbed, globular cacti ''Echinocactus''. This text is intended to show the collector what the range of species is and to help in the identification and verification of acquired plants, as well as presenting, in one source, information spanning a number of years. In the course of preparing this work, John Pilbeam increased his collection of this genus from 120 plants to 800, and has travelled to see collections worldwide.