Chapter 3 considers the meaning of an ‘act of aggression’, defined in Article 8bis (2) of the Rome Statute. It opens with a comparative analysis of the terminology employed in the UN Charter to describe inter-State armed force. It then summarises the shortcomings of the Definition of Aggression annexed to General Assembly (GA) Resolution 3314 (1974), which provided a basis for Article 8bis (2). Among other things, it is demonstrated that the 3314 Definition does not reflect the understanding of an act of aggression seen in the practice of the UN Security Council, GA, or the International Court of Justice. It is also explained how some of the 3314 Definition’s shortcomings were exacerbated by the selective use of its provisions in Article 8bis (2). In addition, the chapter considers the treatment of the acts of non-State actors, and cyber operations and emerging weapons technologies. Finally, the chapter analyses the status of the ‘Understandings’ adopted by the ICC Assembly of States Parties alongside the aggression amendments, and their effect on our understanding of the definition of the State act element of the crime.