The amicus curiae practice pertains to the debates on participation in national and international proceedings. However, how can the amicus curiae procedure affect international criminal justice? In this book, Sarah Williams, Hannah Woolaver, and Emma Palmer explore how the amicus curiae mechanism in various international criminal tribunals could contribute to the legitimacy of institutions and their decisions.

This book consists of eight chapters. Chapter 1 defines the notion of legitimacy with indicators and analyses of how the amicus practice links to legitimacy. In this book, legitimacy covers both normative legitimacy and sociological legitimacy. The book defines normative legitimacy as “the justification of authority” of courts to decide cases, whereas sociological legitimacy evaluates “whether the institution deserves support” (p. 5). The legitimacy perspective is timely for discussions on the International Criminal Court (ICC) and its decisions. Based on a functional approach, the authors describe three potential functions of an amicus curiae; namely, their expertise, as well as representative and communicative functions (p. 18). From a comparative perspective, Chapter two studies the amicus in national jurisdictions and other selected international institutions. This informative chapter provides a big picture for understanding the core of the amicus practice. It also highlights how fair trial rights operate as a constraint on the amicus in criminal proceedings.

Chapters 3 and 4 analyse the legal framework of the amicus curiae in six international criminal tribunals (the ICC, the International Criminal Tribunal for the Former Yugoslavia, the International Criminal Tribunal for Rwanda, the Special Court for Sierra Leone, the Special Tribunal for Lebanon, and the Extraordinary Chambers in the Courts of Cambodia) and the amicus practice in these selected tribunals between 2014 and 2019. Chapter 3 introduces the mechanism, the normative provisions and restrictions on the amicus practice, and other alternative mechanisms that perform the functions of amicus curiae. Chapter 4 covers the amicus practice through an empirical study of amicus curiae applications and briefs. Limited evidence shows that amici had directly affected the outcome of the tribunals’ decisions. The authors argue that the amicus practice is most able to contribute to the legitimacy of the tribunals (pp. 159–65). Based on the legitimacy framework and the typology of the amicus curiae function, this unprecedented and systematic empirical study provides readers with a comprehensive overview of the practice and satisfactory explanations. The methodology adopted (Annex A) helps readers avoid getting lost in a detailed descriptive analysis of cases. However, in doing so, the cutting-edge legal questions at these tribunals are less discussed.

Within chapters 5 to 7, the authors carefully analyse how civil society, the defence, States, and international organisations may enhance (or undermine) the legitimacy of tribunals, in particular the ICC, by fulfilling the three functions of the amicus practice. In conclusion, the authors suggest that the ICC and other tribunals should manage the amicus practice within the legitimacy framework. The authors have also provided a valuable Practice Guide for potential civil actors’ participation and Chamber's management (Annex C).

This book is an exceptionally high-quality and well-organized work about the role played by the amicus among other critical stakeholders in promoting legitimacy for international criminal tribunals. The lens of legitimacy provides an analytical structure for further empirical study. With its clear exposition, this book is an informative and substantive addition to the literature. It should be of interest to international criminal tribunals, civil actors, and persons professionally involved with international (criminal) law.