Abstract
A review of the known diploid species in Hieracium L. s.s. (Asteraceae, Cichorieae) is presented. This article aims to summarize the current knowledge of the taxa with the basic ploidy level in the genus (2n = 2x = 18), as these entities are supposed to have originated the outstanding diversity observed nowadays, which is largely ascribable to polyploid microspecies. The study of extant diploid species is crucial for the understanding of the speciation dynamics that occurred in hawkweeds. All available cytological data in the literature, pertinent to Europe and adjacent countries, are included to have an overview of the obligately sexual species in this genus and highlight gaps and uncertainties. In order to preliminarily investigate the geographical distribution, all records are georeferenced and projected on a map to highlight the hotspots of Hieracium diversity. A brief account of each taxon is included, with some additional considerations and remarks on doubtful records.