This is a complete revision of this Central and S. American genus, well-known for its cultivated species. The main body consists of a taxonomic revision (text in German, descriptions in Latin); 39 species are keyed out, only one is new, there are I new combination and several new varieties, the latter mostly based on former species; a number of former species have been recognized as hybrids. Localities are very accurately given, often latitude and longitude are added. General chapters include ecology, pollination, palynology (by Dr. Punt), phytochemistry (by Dr. Hegnauer & Dr. Kubitzki), and chromosomes. At the end natural hybrids and those found in gardens are listed, concluded by a listed evaluation of taxa and cultivars found in cultivation. The author concludes that the genus is very homogeneous, also in pollen and chromosomes. This appears also from easy hybridization in which at least 10 species are involved, in culture sometimes even species which are in nature geographically isolated. And hybrids have at least in certain cases proved to be fertile. Even triple hybrids have been found. Because of the very large amount of material studied the species populations and their ranges have become rather clear and hybridization occurs where populations come into contact. From this the author deduces his opinion about the hybrid status of certain specimens. In one biotope only one species occurs and the species are hence replacing either geographically or ecologically. This is obviously comparable to the situation in the genus Geum.