In 2019, in the case of Prosecutor v. Al Hassan, Prosecutor Bensouda sought confirmation of the first charge of gender-based persecution at the International Criminal Court (ICC). Given that no previous international tribunal had jurisdiction to prosecute persecution on ‘gender’ grounds, this was an important first step towards developing the jurisprudence on this crime. Meanwhile, the Rome Statute’s definition of ‘gender’ has been discussed and debated in the International Law Commission (ILC) in the context of the ongoing development of draft articles on crimes against humanity. In the context of these recent developments in the ICC and ILC, this article closely examines the Rome Statute’s definition of ‘gender’, and explores what ‘gender’ means as a ground of persecution. We conclude that as a result of the definition agreed to at the 1998 Rome Diplomatic Conference, the Rome Statute’s definition of ‘gender’ captures the social aspects of gender, consistent with widely accepted practice at the time of the Statute's adoption and in the two decades since. These are important facts to hold on to as the focus on the definition of ‘gender’ intensifies, the ICC’s jurisprudence on gender-based persecution evolves, and efforts advance to develop a new convention on crimes against humanity.