This contribution deals with the phenomenon of foreign fighters in Ukraine. The author analyses risks and threats to the security of NATO countries connected with the activities of such fighters on the territory of Ukraine (including the separatist republics) and their return to their home countries. The development of the foreign fighter issue during the so-called Ukrainian crisis is described. Significant risks and threats connected with their engagement in the conflict and return home (i.e., propaganda, involvement in paramilitary units, threatening political opponents, etc.) are identified and assessed.