One of the major requirements of the international humanitarian law is the provision in regard of the fact that all the population of a state in war cannot participate in a military conflict, this task is performed by special structures created by the state: regular and irregular armies, usually united by the term “armed forces” (combatants and non-combatants).In the article there is offered an analysis of customs, international legal documents, writings of Russian and foreign scholars, state practice, which reflect the evolution of the terms combatants and non-combatants during a substantial period of time starting from conclusion of the Convention on institutes and customs of land warfare of 1899/1907 and till Additional Protocol 1 of 1977 to Geneva Conventions of 1949. The author comes to a conclusion that Additional Protocol 1 clarified the issue of determining rightful participants in a military conflict as pertaining to one of the categories: a) combatants – persons belonging to armed forces of a state, who are entitled to wage a war; b) non-combatants (medical workers, priests) from the body of armed forces, who do not have such a right. Nonetheless the author points out that there is still no single opinion on this issue.