Abstract: In this paper, we examine the actual and potential roles of silence as an element of jus ad bellum treaty law and customary international law. By silence, we mean a lack of a publicly discernible response either to conduct reflective of a legal position or to the explicit communication of a legal position. We focus here on the silence of States and the United Nations Security Council as the primary actors who are positioned to shape, interpret, and apply jus ad bellum. We evaluate how silence has been employed by various scholars in making legal arguments in this field, and how silence may have the potential to affect the formation, identification, modification, and termination of various doctrines. We submit that there is no quantum of silence that has clear doctrinal force. We argue that, at least in relation to jus ad bellum, only certain forms of qualified silence — whether of States or of the Security Council — may be capable of contributing to legal effects. We further contend that, due in part to the nature and status of the norms underlying this field, those forms of qualified silence ought not to be lightly presumed. Arguably, there is a strong, if rebuttable, presumption that silence alone does not constitute acceptance of a jus ad bellum claim. Still, States and other international actors should be aware of the possible role that their silence could play in the identification and development of this field. We complement our analysis with an Annex that offers the most comprehensive catalogue to date of communications made by U.N. Member States to the Security Council of measures taken in purported exercise of the right of self-defense. The catalogue records over 400 communications made since the founding of the United Nations in late October 1945 through 2018. These communications reflect the views of the submitting State(s) on the scope of the right to employ force on the purported basis of self-defense.