The present study deals with the systematics and taxonomy of the genus Origanum (Labiatae, Saturejeae). As this difficult genus was never before monographed, a revisional study was much needed. The data presented are mainly based on the study of herbarium specimens and in some cases of living ones. The picture was completed, as far as possible, with data from various literature sources. A short survey is given of the taxonomic history of Origanum, which goes back as far as Linnaeus, and shows that genus and species concepts of various authors have much differed. A morphological outline of Origanum is given, from which it can be concluded that most generic characters are rather variable. Origanum is characterized in the following ways. Medium sized, subshrubby Labiatae, rich in volatile oils, with subsessile, ovate, glandular punctate leaves and paniculate inflorescences; few flowered verticillasters arranged in (dense) spikes with distinct, often coloured, bracts; calyces variable: 5-toothed, subregular or 2-lipped or 1-lipped, with developed or reduced teeth; corollas 2-lipped, sometimes saccate or flattened. Origanum is compared with related genera found in the subtribes Melissinae and Thyminae within the tribe Saturejeae. One conclusion is that there are no arguments to maintain these subtribes. Further it can be concluded that Origanum ’s nearest relatives are Thymus, Satureja and Micromeria. In the sections Campanulaticalyx and Elongatispica, Origanum comes near to the latter genus. The genera Satureja and Micromeria, which together contain the bulk of the species in the group, are in need of a revision. When this is carried out it may become clear that several genera should be redefined, including possibly Origanum. The genus is divided into 10 sections, of which two are new and one transferred from another genus. In all 38 species are recognized. Specific differences are found in the indumentum and in the size and/or shape of spikes, bracts, calyces, corollas, and filaments. These and other characters are uniformly included in the descriptions given. In two species infraspecific taxa are listed. In addition 17 hybrids are recognized, of which four are new and three others were previously described as species. For six taxa a new status is introduced (in one case in a new combination), while two new combinations are made, one species name is validated, and one new name is given. Type specimens are recorded for all taxa and identification keys to all taxa are given. Important characters are picutred for all species and infraspecific taxa. Distribution maps are given. The chromosome number of four species of Origanum is known at the moment. In all cases (apart from a few counts for O. vulgare) the number 2n = 30 is established. Gynodioecy occurs in the species of five sections. Most Origanum species (c. 70 %) are found in the East Mediterranean subregion, while a few species occur in the West Mediterranean subregion. Most species occupy (rather) small areas: c. 70 % is endemic to one island or mountain (group). Only O. vulgare has a very large area, ranging from the Azores to Taiwan. Origanum species usually inhabit mountain regions and rocky places with calcareous stone. Though hybridization is frequently found in Origanum, hybrids do not usually occur in large numbers. It is postulated that not only intra-, but also inter-generic hybrids occur in this group of Saturejeae. In a hypothesis for speciation hybridization is seen as the most important way of origin of Origanum species. This hybridization can have taken place between species of Origanum as well as between Origanum species and species from related genera. Origanum species are generally rich in volatile oils containing considerable quantities of carvacrol and thymol. Since ancient times species of Origanum are used as medicinal herbs but nowadays this is of minor importance. In the course of time the use of Origanum species as culinary herbs has become more important. In recent times several species of Origanum have also been used as ornamentals. Two Puccinia species and a strain of alfalfa mosaic virus are found as parasites on Origanum.