While existing scholarship has analyzed State silence through frameworks such as acquiescence, this study advances a distinct perspective by focusing on the normative force of ‘legitimate or reasonable expectations’ created by passive conduct. It examines how silence binds States under international law by generating such expectations, thereby giving rise to legal obligations. Departing from traditional frameworks treating silence merely as passive behavior without any legal value, this study demonstrates that when circumstances objectively call for a reaction, a State’s silent conduct – such as silence or a lack of objection – creates a ‘legitimate or reasonable expectation’ in other States that the silent State has accepted a particular legal position. Such expectations crystallize into obligations when other States rely on this passivity, through principles of good faith. Consequently, the silent State is obliged to adhere to the agreements or understandings tacitly shaped by its passiveness. The analysis integrates doctrinal scholarship with international case law to show how silence shapes rights and duties across international law. By anchoring acquiescence and estoppel in the bedrock principle of good faith, the article advances a coherent theory of how passive conduct becomes a source of obligation, reinforcing the stability of international legal relations.