The genus Didymodon belongs to the family Pottiaceae, and is represented by approximately 122 species (Zander 1993) distributed throughout the world and found on a variety of substrates, mostly rock or soil.
Separation of species in this genus is mainly on gametophyte characters, in part because of the few collections with sporophytes. Didymodon is dioicous in all cases in which the sexuality of the species is known. The number of diagnostic characters is low, which means that Didymodon has often been considered a complex and taxonomically difficult genus.
The systematic position of the genus Didymodon has been controversial. Some authors have treated it as a synonym of Barbula (e.g. Dixon 1924, Mönkemeyer 1927, Hilpert 1933, Chen 1941, Smith 1978, Nyholm 1989, and Kürschner 2000). The taxonomic differences between Didymodon and Barbula have long been based on peristome characters. This made some gametophytically very similar species included in different genera (e.g. D. acutus and D. rigidulus). Saito (1975) was the first author to define the genus Didymodon on basis of gametophytic characters. He considered the axillary hairs of the leaves as the most important character to separate Barbula from Didymodon. Barbula has entirely hyaline axillary hairs, while in Didymodon the hairs have one or more brown basal cells. Besides this, some Didymodon species have recently been treated in segregated genera Geheebia Schimp., Husnotiella Cardot, and Trichostomopsis Cardot. Zander (1978a, 1993) extended the definition of Didymodon given by Saito (1975) and synonymized these three genera under Didymodon. Recent molecular data obtained by nuclear ITS1 and ITS2 gene sequences show that the genus Didymodon and these segregate genera form one monophyletic group that is clearly separated from Barbula (Werner et al. 2005).
Until now, a formal worldwide revision of Didymodon has not been available. Prior to this work, only the species of section Fallaces of Didymodon have been revised in the study area of this work (Jiménez et al. 2005), although Didymodon had previously been treated in regional floristic-taxonomic treatments or checklists. For Europe, Düll (1984), Düll-Hermanns & Düll (1985) and Ku?era (2000) treated Central European taxa; Ku?era (2002) studied North European taxa, and Jiménez (2004) treated taxa from the Iberian Peninsula. Also, no study included North African taxa or those from Central and Southwest Asia. Many more works have been published for this genus in North America and Mexico (Zander 1978a, 1981, 1994, 2002), Central America (Allen 2002), East Asia (Chen 1941, Li et al. 2001), and Japan (Saito 1975, Noguchi 1988).