This book examines the UK approach to investigating international crimes and serious human rights violations. In 2010, the United Nations Secretary General referred to the emerging system of international justice, including the creation of the International Criminal Court, as the 'Age of Accountability.' However, the UK has sometimes struggled to comply with its international law obligations. Using examples from the post-World War II period to 2018, interviews with leading UK military lawyers and newly disclosed official documents, this work explains the legal duties, how the UK military and civilian justice systems investigate alleged military misconduct and highlights the challenges involved. It provides suggestions on strengthening domestic law and policy and its importance for the UK's legitimacy as an exporter of rule of law expertise. This text is essential reading for practitioners, academics, government officials and students of international, criminal, humanitarian or human rights law.