The barley genus, Hordeum L., with around 40 taxa, has usually been considered a homogeneous biological group. Newer information has revealed a more heterogeneous picture. The traditional circumscription of the genus is now controversial, and Hordeum in the traditional sense probably consists of 2–4 groups of species. Data on interspecific hybridization, genome relationships, nuclear and cpDNA variation, isoenzymes, and monoclonal antibody specificity, all show a similar picture, but a picture that differs from the classification based on morphology. Most results indicate that H. vulgare subsp. vulgare (cultivated barley) and its progenitor, subsp. spontaneum together with the perennial H. bulbosum and possibly the annual weed, H. murinum s. lat. constitute one group. Most other species, with the exception of at least H. marinum, are relatively closely related and share common genome(s). The interpretation of phylogenetic patterns is complicated by different types of polyploidy, and meiotic pairing regulation.