Common Article 1 (CA1) to the Geneva Conventions of August 1949 and Additional Protocols I and III, imposes obligations on High Contracting Parties to respect and to ensure respect for international humanitarian law (IHL), in all circumstances. This obligation contains two dimensions: the internal component, which requires States to internalise the obligation in their governmental and private activities, and the external dimension, which requires States to undertake all measures to ensure that other States respect IHL. This chapter argues that the establishment of the International Criminal Tribunal for the former Yugoslavia and the International Criminal Tribunal for Rwanda constituted an example of third States complying with CA1. This consequently served as the precedent for third States to further ensure respect for IHL through participation and cooperation with the International Criminal Court. It then identifies the establishment of the hybrid courts and more recently, quasi-hybrid courts, such as the Extraordinary African Chambers and Kosovo Specialist Chamber, as new developments in the use of international criminal courts and tribunals in fulfilment of CA1 obligations.