As an introduction to a series of contributions on the genus Gibbaeum N.E.Br., a review of the current taxonomic knowledge of the genus and open problems is given. The second part clarifies the intricate taxonomic history of Gibbaeum haagei. Gibbaeum haagei Schwantes ex H.Jacobsen 1935. was invalidly published and is a synonym of G. petrense (N.E.Br.) Tischer. Gibbaeum haagei Schwantes 1938. was validly published and misidentified by Glen (1974) and subsequent authors, as the white-tomentose Gibbaeum species from the Swellendam region (Western Cape Province, South Africa). Glen wrongly treated G. haagei Schwantes 1938. as a later illegitimate homonym of the earlier G. haagei Schwantes ex H.Jacobsen 1935. nom. inval. and proposed the new species G. austricola Glen (‘austricolum’) nom. inval. for the whitetomentose species from the Swellendam region. This prompted Hartmann (2001) to publish G haaglenii H.E.K.Hartmann as a replacement name which is illegitimate. Here it is shown that G. haagei Schwantes 1938 as well as G. haaglenii are synonyms of G. velutinum (L.Bolus) Schwantes. The white-tomentose species from the Swellendam region was published as G. haagei var. parviflorum L.Bolus, but is as yet without a valid name at the species level. As a new starting point, it is described here as G. hartmannianum Thiede & Niesler sp. nov.