In Common Article 1 of the 1949 Geneva Conventions, States Parties undertake to “respect and ensure respect” for the Conventions. This article focuses on the question whether an interpretation of that provision leads to the conclusion that it contains an external element. The term “External element” refers to an obligation for States to ensure respect for the Conventions not only internally (i.e., by their nationals as a whole), but also by other States, and possibly even by organized armed groups involved in extraterritorial non-international armed conflicts. The article applies the rules of treaty interpretation, as codified in Articles 31 – 32 of the Vienna Convention on the Law of Treaties, to common Article 1, to determine whether there is such an element. It argues that a narrow focus on the travaux préparatoires does not fully take into account all the elements that play a role in the interpretation of a treaty provision. The article finds that an interpretation of common Article 1 does not necessarily lead to an unequivocal answer to the question addressed. Taking all the elements of treaty interpretation together, however, it concludes that common Article 1 does include an external element. This conclusion is based, inter alia, on State practice that has not previously been taken into account.