Lecture summary: The role of civil society in drafting and the adoption of the Rome Statute for the International Criminal Court (ICC) is well known, as is the contribution of civil society to advocating for states to ratify the Statute and implement its provisions. However, despite the importance of these contributions, such opportunities do not constitute direct participation in the formal proceedings of the ICC. Other than the role of civil society actors as a witness, be it as an expert or a factual witness, there is only one option for direct participation of civil society in ICC proceedings: that is, to participate as an amicus curiae. States, too, have more limited rights of participation in proceedings before the ICC, particularly in comparison to other international institutions. However, the proceedings may raise issues of direct relevance to a state or broader relevance to several states, including states parties and non states-parties. Where the Rome Statute legal framework does not provide for formal rights of participation for states, states too must rely on the amicus curiae mechanism. This lecture addresses the practice in relation to the amicus curiae in proceedings before the ICC, particularly the reliance on this mechanism by both civil society actors and states. It sets out the legal framework for the amicus curiae in the ICC, analyses the types of actors that have sought to appear as amici curiae, and examines the process and criteria applied by ICC Chambers when considering applications, as well as the range of topics on which amici have sought to make submissions. It concludes that the ICC has so far been cautious in its approach to the amicus curiae; however, amici – particularly states – can influence outcomes at the ICC.