Contemporary international lawmaking is characterized by a rapid growth of “soft law” instruments. Interdisciplinary studies have followed suit, purporting to frame the key question states face as a choice between soft and “hard” law. But this literature focuses on only one form of hard law — treaties — and cooperation through formal institutions. Customary international law (CIL) is barely mentioned. Other scholars dismiss CIL as increasingly irrelevant or even obsolete. Missing from these debates is any consideration of whether and when states might prefer custom over treaties or soft law. This article applies an instrument choice perspective to demonstrate custom’s continuing relevance to contemporary international lawmaking. First, we use instrument choice to identify the distinctive design features that distinguish CIL from treaties and soft law. As an ideal-type and in comparison to conventions and nonbinding norms, custom is a non-negotiated, unwritten and universal form of cooperation. Second, instrument choice illuminates the constraints that limit custom to particular types of cooperation problems, which we label as custom’s “domains.” Specifically, CIL’s design features limit custom to situations in which all-states-benefit from a norm, hegemonic custom, and normative custom. Third, an instrument choice approach predicts that states will continue to prefer custom over treaties and soft law when its design features or substantive norms offer advantages over international agreements and nonbinding norms. Custom thus retains contemporary relevance even in an age of soft law and treaties, so long as states act within the constraints imposed by custom’s domains.