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The media coverage of irregular armed groups during the conflict in Ukraine has spurred many comments about the presumably new forms of warfare that, used by Russia, characterize the 21st century. However the use of irregular forces, which have no official military status, is far from being new in the history of warfare, and it is even normal since the end of WW II. To contribute to the discussion on irregular armed forces, this article analyzes the origins of the phrase in military and academic literature and presents a short historical description of the various forms of this irregularity. These remarks are then brought to bear on post-Soviet conflicts, in particular in Transnistria and the Donbas.