Qui tacet consentire videtur si loqui debuisset ac potuisset v. quo tacet neque negat neque utique fatetur (he who remains silent is considered to consent, if he must and can speak v. he who remains silent neither rejects nor accepts-affirms)

The present work aims to analyze silence/acquiescence as a legal fact that constitutes a passive conduct and produces legal effects. The paper is divided into three parts. The first analyzes silence as a legal act in international law, translates the phenomenon of acquiescence and elaborates the relative doctrine and jurisprudence as well as the limits that this term presents. We continue with the acquisitive prescription and the silence in the formation of the treaties. It compares acquiescence in dispute resolution as well as the establishment of jurisdiction of international courts and tribunals. The waiver applies to the area of international responsibility of a State and the decision-making mechanisms within the opting-out procedure. Silence as a legal fact in stricto sensu integrates a qualified conduct to which the production of legal effects compose an institution of international law. The production of their legal effects contemplate the existence of an involuntary silence such as l'estoppel by silence and the extinguishing prescription. The jurisprudence from the ICJ and from arbitral awards is extensive and helps to better know and understand the institution of acquiescence and the importance that the silence of States has in contemporary international law.