The UN General Assembly established the International Law Commission (“ILC”) in 1947 to assist States with the promotion of 1) the progressive development of international law and 2) its codification. The ILC’s first assignment from the General Assembly was to formulate the Nuremberg Principles, which affirmed the then radical idea that individuals can be held liable for certain international crimes at the international level. Since then, the ILC has played a seminal role in the development of modern international criminal law. In 2017, the ILC adopted on first reading a draft convention aimed at the prevention and punishment of crimes against humanity which it transmitted to States for comments. The draft treaty will help fill the present gap in the law of international crimes since States criminalized genocide in 1948 and war crimes in 1949, but missed the opportunity to do so for crimes against humanity. This Article examines the first reading text using the lens of the ILC’s two-pronged mandate. Part II explains how the ILC can take up new topics and the main reasons why it decided to propose a new crimes against humanity convention. Part III discusses positive features of the draft convention, highlighting key aspects of each of the Draft Articles. Part IV critiques the ILC draft treaty focusing on inconsistencies in the use of the ICC definition of the crime, immunities, amnesties, and the lack of a proposal on a treaty monitoring mechanism. The final part draws tentative conclusions. The author argues that, notwithstanding the formal distinction drawn by the ILC Statute between progressive development, on the one hand, and codification, on the other hand, the ILC’s approach to the crimes against humanity topic follows a well settled methodology of proposing draft treaties that are judged likely to be effective and broadly acceptable to States rather than focusing on which provisions reflect codification and which constitute progressive development of the law. It is submitted that, if the General Assembly takes forward the ILC’s draft text to conclude a new crimes against humanity treaty after the second reading, this will make a significant contribution to the development of modern international criminal law.