This chapter reviews the main international humanitarian law (IHL) issues that arose during the Vietnam War and how those issues were addressed in subsequent development and codification of this law. The ‘Vietnam War’ refers to the armed conflict that took place between, on the one hand, North Vietnam and the National Liberation Front (NLF, an armed group formed in South Vietnam in 1960) and, on the other hand, South Vietnam, the United States, and their allies during the 1960s and the 1970s. The conflict had a lasting impact on IHL in many respects due to the gravity of the humanitarian issues that arose during the war. First, at the most fundamental level, many issues arose in determining whether IHL was applicable in the first place and, if so, which IHL rules applied. The conflict also raised the question whether the IHL rules in force at the time were adequate in protecting civilians and civilian objects and regulating the means and methods of warfare that were employed during the conflict. The conflict further highlighted the issue of how combatants who were captured while fighting clandestinely should be dealt with in IHL.