This Companion is a one-stop reference resource on the Phnom Penh based `Khmer Rouge tribunal'. It serves as an introduction to the Extraordinary Chambers in the Courts of Cambodia, while also exploring some of the Chambers' practical and jurisprudential challenges and outcomes. Established by an agreement between the United Nations and the Government of Cambodia, the tribunal has been operational since 2006, and seeks a mandate to try those most responsible for serious crimes committed during the Khmer Rouge period from 1975 to 1979. The Companion is organized around a series of themes including legality, structure, proceedings, jurisprudence, legitimacy and legacy, and offers both direct insights and academic analysis by an author who has worked as senior adviser to the tribunal's Pre-Trial and Supreme Court Chambers. This original book will prove a valuable and stimulating read for lawyers, judges and UN staff working within, establishing, or monitoring international courts and tribunals as well as local and international NGOs in Cambodia concerned with accountability for the crimes of the Kymer Rouge era. Academics focusing on international criminal justice will also find this Companion useful to assess the contribution of the Extraordinary Chambers, both during the tribunal's lifespan and after it has closed its doors.