At the most fundamental level, irregular or guerilla warfare is no different from conventional warfare as a strategic problem, that is, as a bridge between means/tactics and political ends. Thus, the just conduct of asymmetrical warfare would seem to be no different than for conventional warfare, unless something unique to asymmetric conflicts requires the jus in bello principles of noncombatant immunity and proportionality to be waived or radically altered. Jus ad bellum conditions are, at first glance, more problematic, because the first criterion for the permissible use of force in the just war tradition was that it be authorized by public authority. This chapter examines these issues with a focus on how non-traditional combatants have been classified into legitimate and illegitimate classes, and the ramifications for this in customary and contemporary international law and practice.