The four known species of Tarnania Tuomikoski are reviewed, viz. T. dziedzickii (Edwards, 1941), T. fenestralis (Meigen, 1818), T. nemoralis (Edwards, 1941), and T. tarnanii (Dziedzicki, 1910). Males and females are re-described, photographed and their terminalia figured. An attempt is made to homologize and name specific substructures of the male gonostylus, widely recognizable within the tribe Exechiini. A phylogenetic analysis of the Tarnania species, including representatives of the six other genera of the Rymosia s. lat. genus group, strengthens the view that Tarnania is monophyletic. Tarnania is supported by six unambiguous character changes of which three are considered to be unique synapomorphies, viz. 1) lower parts of metepisternum and metepimeron covered with campaniform sensilla; 2) hind tibia with several curved bristles posterodorsally, placed irregularly in more than one row; and 3) anterior branch of male gonostylus forming an elongated, bifurcated lobe. The most parsimonious hypothesis for relationships among the species of Tarnania is ((T. tarnanii + T. nemoralis) + (T. dziedzickii + T. fenestralis)). The practice of regarding the genera of the Rymosia s. lat. genus group as an assemblage of plesiomorphic genera is also supported, the most parsimonious hypothesis for relationships among them being (Notolopha (Allodiopsis (Myrosia (Synplasta (Tarnania + (Rymosia + Pseudorymosia)))))). Tarnania is most diverse in northwestern Europe, with all four species known from Norway, Sweden, Great Britain, Belgium, Germany, Switzerland, and France. T. dziedzickii extends its distribution southwards to include the North African and the Near East Regions. T. fenestralis is widely distributed in the western Palaearctic Region and extends its distribution eastwards to the eastern Palaearctic region. Only T. tarnanii displays a wide Holarctic distribution including most of Russia, Alaska, Canada and Greenland.