Maihuenia is a small genus of low cushion-forming, frost-resistant, highly mucilaginous cacti bearing terete, persistent leaves, and areoles with normally one central and two short lateral spines. The flowers are white to yellow, with numerous bract-scales on the receptacle and inferior gynoecium, the fruits are subglobular to clavate, hollow, with fleshy fruit wall, thin ovary roof, and numerous black seeds. The distribution of the genus is south Andean-Patagonian, ranging from south central Chile and western Argentina to southeastern Patagonia (Argentina). Only two species, Maihuenia patagonica, and M. poeppigii are recognized, and ten specific names are listed as synonyms. Typification of the names is provided. This study is based on herbarium and liquid-preserved material as well as on field studies and observation of both species in cultivation. The introductory chapters treat the historical background (including collecting, introduction, and taxonomic history), morphology of roots, stems, leaves, flowers, fruits, and seeds, seed germination and seedling development, anatomy, chromosome number, pollen morphology, distribution, phytogeography, ecology (including geology, climate, heat resistance, frost resistance, phenology, pollination, dispersal, reproduction and regeneration, predators and other animals observed), conservation, uses, cultivation, and intergeneric relationship and evolution. The systematic treatment includes a key to the species, typification of all names, synonymy, descriptions of genus and species, distribution and phenology, detailed specimen citations, notes on taxonomic history as well as illustrations and distribution maps. A list of exsiccata, of vernacular names, and a name index are provided.