The US has been involved in the majority of transitional justice measures established since the 1990s. This study explores this phenomenon by examining the forces that shape US foreign policy on transitional justice. It first investigates US influence on the evolution of the field, and then traces US involvement in three illustrative cases in order to establish what US involvement entails, why the US gets involved and how the US has impacted individual measures and the field as a whole. The cases include: the Khmer Rouge Tribunal in Cambodia; the trial of Liberian President Charles Taylor and the Liberian Truth and Reconciliation Commission; and the Justice and Peace Process in Colombia. These cases represent different transitional justice measures, transition types and geographic regions – all key dimensions in the field. These measures were also all established in the 2000s, a period which reflects a different historical moment in the field’s evolution. The cases shed light on the actors who play a key role in the field – from presidential administrations to Congress to the State Department and others. The study is based on nearly 200 interviews and archival research undertaken in the US, The Hague, Cambodia, Liberia and Colombia, providing a strong basis on which to draw conclusions about US foreign policy on transitional justice.