The Article proceeds in four parts. Part I introduces the Rome Statute’s provision on nullum crimen sine lege, focusing in particular on its requirements that judges strictly construe crime definitions, construe ambiguous provisions in favor of defendants, and avoid crime creation by analogy. It offers working definitions for relevant concepts and describes some of the difficulties in applying them, particularly in light of the Rome Statute’s provision setting out the sources of law the court is to consider. Part II asks whether strict construction makes sense in the context of international crimes. It assesses the values that undergird the principle, most importantly, notice, separation of powers, the judiciary’s role in protecting individual freedom, efficiency, and democratic accountability. It concludes that the justifications relied on in domestic jurisdictions for strict construction apply more readily in the international context than one might think, but suffer from many of the same flaws they do domestically. These flaws are often magnified at the ICC. Part II also examines justifications for strict construction that are particular to the ICC, including promoting human rights, respecting state sovereignty, encouraging participation in the ICC framework, and ensuring that the ICC focuses its limited resources on the gravest crimes. Ultimately, this Article finds merit to these arguments, but not enough to prioritize lenity over competing language in the Rome Statute itself and over other tools of interpretation. Part III assesses the potential ordering for strict construction in light of the purposes it serves. Part IV then offers a conception of strict construction that distills it to a few critical principles that better support the justifications for it and help to square strict construction with the realities of ICL and the Rome Statute. These principles are: avoiding usurping the authority of states, avoiding unfair surprise to defendants, and seeking, where possible, to clarify ICL. This proposed conception of strict construction grapples with the inescapable fact that the great legality challenge of ICL likely is not ambiguity, but rather vagueness.